windows xp boot progress bar very slow and OS won't start

What viruses were found and by what scanners?

A repair of windows may help
http://michaelstevenstech.com/XPrepairinstall.htm

Not sure, which files could be deleted during the scan process.
You can try to boot from the XP CD to make a repair of your windows installation, so you can safely do a backup of your important data.
After you make a backup, you can reinstall Windows, as if you're got infected, you'll never be safe without reinstallation.

Please define 'scan it in another system for viruses':

Which program did you use?
Did it find anything?
Did you make any changes to the contents of the drive before/after the scan was completed?

Thanks!

you don't happen to have like 1 million files directly in your C:\ drive?
Had this issue recently... let it boot for an hour or two to see if it finally comes up.

Used ESET, Malwarebytes, and Super antispware I usually use thes no issues. I run a computershopt BTW. When I attempt to repair windows same thing happens during install....

I also backed up the disk no problem before the above exercise so I don't think it's a faulty drive.

Have you tried accessing the backup to see if it completed successfully and is accessible?

I know its a lame suggestion but have you tried running the basic diagnostic test through BIOS on the HDD?

Can you boot to safe mode?

Yes backup is fine, there is no option in bios for HDD test. Can boot into safe mode however because I've already started a repair install it won't let me operate in safe mode it reboots and tries to run the repair again.