avatarp,
I am currently running Office 2000 and Office XP on my Windows 2000 workstation. Office 2K was installed first, and when I installed Office XP I did a custom install and directed it to My documents (technically, C:\Documents and Settings\username\My Documents\OfficeXP\Office10\).
The only problem I've encountered thus far is that at times XP will override as default office program if i double-click on a file icon. I keep the 2K shortcuts on my quicklaunch bar and the XP icons on my desktop, and each have their own folders in my Start Menu.
Good luck!