>>I've downloaded VM Workstation & ThinApp and was wondering if that's sufficient to run the ThinApp application in?
Yes that should be fine.
>>For some reason, my PC is a few years old
ThinApp as such does not need a very powerfull machine for its own, it all depends upon the application you are capturing. Workstation on the other hand, needs a machine with good RAM to start with. (I ran workstation on a 3 year old C2D machine with 8 GB RAM and it runs fine).
>>Can anyone point me in the right direction to get started with Thinapp and packaging applications?
Office is a complex appication to start with. I suggest you take a basic free app (like notepad++), package it and get the experience. Than move to office. (should not take much time though).
Check this for office http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=102228...
>> Also does ThinApp run inside any particular O/S?
It needs windows 2K+. (google for its manual, that has all these basic answers)
Good Luck