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tested...

I know that a "normal" XP will install on a VPC 5 and 6, and it works fine. I tried it on my Dual 1.25, giving the "guest OS" 256 MB RAM. But all those graphical artifacts (like OS X, but oh, so ugly) slows the experience down....
I also have tested VPC 6 with Win 2000 SP3 pre-installed, vs VPC 6 DOS 2000, then installed an original Windows 2000 CD on it. No difference. Only the pre-installed version turned many visual effects off, which made it snappier. But after I did the same on my own installation, there was no differnce.
Side note: Using the DOS 2000 version, no VPC additions are installed. To do this you need to extract the disk image of this installer which is inside the Resources of the Virtual PC app. Then you must assign the image to VPC.
 
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