I accidently did this a few weeks back and here is the answer I got from another member here at BetaONE.
If you happened to accidently install 2000 after you installed XP, here is how to fix it.
Plop in your XP cd and boot off it. Go into the recovery console. When you get to the command prompt, type 'fixboot' and then yes.
Then type 'cd\' 'attrib -h -s -r ntldr' 'attrib -h -s -r tdetect.com'
Then copy those two files from your i386 folder on your cd to your c drive.
Once copied, exit recovery console and reboot.
This has not been tested by me personally, but it seems like it should work. This is how to recover those files if you hadnt backed them up before you installed 2000. If you are planning to install 2000 and you already have XP installed, the best way is to just back up those two files as DaveH suggested
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