Hi,

It helps if knowing the Windows version of the operating system and workload you are talking about, as not all performance tips will work for the different OS'es or workloads.
However, here's a few links for some tips.

Shut down the logon screen saver in windows

http://blogs.vmware.com/performance/2007/05/windows_vista_p.html

http://www.vmwareinfo.com/2008/07/speed-up-your-windows-2003-servers.html

http://lifehacker.com/5033518/debunking-common-windows-performance-tweaking-myths

Windows Vista® Performance and Tuning



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Wil
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VI-Toolkit & scripts wiki at http://www.vi-toolkit.com

edit: and another one: http://www.yellow-bricks.com/2008/06/20/virtual-machine-tweaks-for-a-better-performance/