Hello Andre,
Thank you very much for your prompt reply! 
I checked the number of snapshots, and you are right.
I had 2 snapshots hidden in the snapshots overview which belonged to the autoprotect function. :smileygrin:
So I deleted one autoprotect snapshot. It only gained me 0,6 Gb of disk space.
Now I have 5 snapshots as far as I can see, but still 44,2 Gb of diskspace.
Even full snapshot removal would not bring me below 40 Gb.
And I am surprised that thedisk space limit of 40 Gb has been exceeded.
What can I do?
Not that this has a direct impact on your issue but why are you still running VMware Fusion 2.0? You should have updated to a later 2.x release as it's free and fixes the issues that the 2.0 release have! 
Anyway the Virtual Machine has 5 Snapshots and if you what to regain any Host Disk Space then you'll need to delete some or all of the Snapshots. Additionally if the issue is not enough free space in the Guest OS then you'll need to delete the Snapshots in order to enlarge the virtual hard disk.
With Time Machine I reloaded an older version of the Virtual Machines file.
It is a known fact that Time Machine is not 100% reliable backing up/restoring Virtual Machines under all circumstances/conditions. Also backing up Virtual Machines via Time Machine is disk/time intensive and wastes a tremendous amount of space for something that may be corrupt and worthless come time to restore it. At a minimum I would exclude Virtual Machines from Time Machine and with the Virtual Machines shutdown, not suspended, and VMware Fusion closed then manually copy the Virtual Machines Package(s) to an alternate location, preferably on to a different physical hard disk. Then keep the User Data that is stored within the Virtual Machine backed up off of the Virtual Machine on a regular basis so as to always have a current User Data Backup. If you have to restore a properly backed up Virtual Machine that is not as current at least you'll have a working Virtual Machine and current User Data to go forward with when you find out your Time Machine Backup of the Virtual Machine fails.
Pitty I cannot upgrade further than 2.0.8.
If the current version is doing for you what you need then I wouldn't upgrade to the next or latest major version unless you had no choice, like if you were running Lion or Mountain Lion then VMware Fusion 2.x is not supported and will not run properly, if at all. As a general rule, I like to go with, if it works then don't fix it!
One last thing that is troubling me is how can the disk size exceed the limit of 40 Gb that is in my preferences now?
Technically a virtual disk cannot grow larger then it is allocated however each snapshot you take has to potential to grow to the allocated size so remember you had 5 Snapshots so theoretically in this case it could consume 240 GB of Host Disk Space but realistically it would probably never reach the theoretical limit. However it would not be unrealistic to consume half that amount until you deleted the snapshots and then shrank the virtual hard disk. In other words with no snapshots and a shrunk virtual hard disk then the virtual hard disk should only consume the amount of Host hard disk space as what's shown take up from within the Guest OS however keep in mind that once shrunk it will continue to grow up to the set size as you continue to use the Virtual Machine even without any Snapshots. Each Snapshot has its own disk that can grow to the set size so the more Snapshots you take the more it adds up and can easily consume more Host disk space then the set size of the virtual hard disk. Hope this makes sense.
Hi Woody and Andre,
Thanks a lot for all your comments.
I followed up all of your advice and now my virtual disk is about 25 Gb. Operation succeeded!
After my virtual disk had grown to 47 Gb, I did the following:
1. I removed all snapshots starting with the oldest.
This will take some time, the reason was explained by you earlier in this discussion.
Some are hidden in the snapshot overview. These are the autoprotect snapshots. Uncheck the "show only snapshots" and you can see them.
Yet the result in disk space was not impressive.
2. I disabled the autoprotect function.
At the time all was OK I made a copy of the file: Documents > Virtual machines > Windows XP Home Edition on an external HDD.
This should be a sufficient back up in case the current version crashes.
Note that first Windows was shut down and then VMWare (not suspended).
Snapshot and autoprotect are great tools if you keep on changing the content of the virtual disk, but they tend to take a lot of space on your HDD.
I do not chage a lot, so a copy of the correctly working virtual disk will do for me.
I use windows for games that cannot be played on a Mac.
3. I cleaned up Windows XP
Removed all programs and files I do not need.
4. Ran clean up disk
This can be found in VMWae Virtual machine > settings > hard disks. Press clean up, even if it says it is not necessary.
This process takes a while, I think it was 1 hour, to complete
5. Ran defrag in Windows.
This also took at least one hour.
6. Ran Shrink disk
Install VMWare Tools, then choose the shrink tab.
The process takes also at least one hour. At one point I got the message that there was not enough free disk space.
Earlier I had set my disk to 40 Gb and the calculated used amount of disk volume was 22. The program then gave a few suggestions to clean up until below 20 Gb. An then it started. After an hour or so it is completed and YES, the size of the virtual machine was about 24 Gb and the 16 Gb free space was regained by the host, Mac. I checked if all was working well, which was the case.
At this point I made a copy of the virtual machine Windows to an external HDD as mentioned under 1.
For me: problem solved.
Thank you guys for all your comments and help. You are doing a great job and you certainly helped me to reduce my computer stress levels.
I learned a lot and the problem is solved. I hope this is also helpful for others.