who said no is an idiot. before you answer be sure first..can't beleive these people have the guts to write wrong asnwers.
anyway my freind yes you can install both xp and vista on the same hard drive with no probs at all, you'll have to restart the pc everytime you want to use a diffrent os
the best way is if your hard drive is partitioned in two equal size partitions, then install the vista like normaly into the other partition where xp isn;t, but don't ever select format just select leave files.
you'll need at least 2gb of ram and each partition about 60gb for best performance. I use 4 diffrent os on the same machine.
you can install both os on the same partition except sometimes you'll find errors as it'll be confusing. but again don't ever select format, always LEAVE FILES AND FOLDERS so you're just adding an os.
when the os are in two diffrent partitions, don't save files or mess with the other just leave it in peace when not in use.
I did xp after i installed vista, but I am sure the otherway around is the same.
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