I have a similar problem:
Had Vista since January, and generally wasn't too worried about it's incompatability with some programs, as with most cases there was a way to get around it. Like most people, I think time simply took it's toll, and I am simply fed up with it, wanting to revert back to XP.
I tried initially just booting from my XP disc so I could use the built in partition manager to format my HDD, only of course came to realise this wouldn't be possible. So I set out for means to do it another way. I cam across Ultimate Boot CD, which I downloaded, wrote onto a disc, and formatted my HDD with, using the inbuilt HDD manager.
After the full format, I ran a full diagnostic, checking for HDD surface problems, as well as problems with the partitions I had made (I made partitions on my second attempt of a Windows XP install, after the first attempt from the full format turned up nothing. Incase it helps, I formatted with NTFS). This diagnostic turned up clean.
Even though I carried out all of the above, and have attempted it all 3 times now (re-installing Vista after every reoccurance ¬_¬ ), I still cannot get past the intial loading screen of the XP boot disc. When it has loaded all the files, and says 'Setup is starting windows', after about a 10 second wait, the dreaded dark blue screen appears, with white writing, stating a critical error has occured, and windows must shut down in order to prevent my computer from being damaged.
I also highly doubt that it is the disc that harbours a problem, since I have tried this with both my XP Home and XP Pro edition discs.
Any help with this would be greatly appreciated - I just really want my XP back. Thanks.