Please help computer reboots

[ugean]

Hello, this is my first time posting on this site so please bear with me. I have used the tools and advice on this site many times but I'm afraid this time I can't find anything that helps. My fathers computer crashed about a month ago. He said he had a bad boot sector. He messed around for two seeks and got all of his information off and installed a new HD. Now for the last week or so it was working fine. Then all of a sudden yesterday it stopped booting. It will count the RAM and get to the windows logo then just reboot. I tried to get into safemode but still it just reboots. I can run the computer using a KNOPPIX 3.8 CD that someone gave me. The problem is I can't get any of the virus scanners to work. It won't let me run the Avg on the HD and none of the online scanners work. I've read some other postings online that say this is most likely a worm or memory related. I do not think its memory because I can run this knoppix off of the CD. Can someone please point me where to go to get his computer back up and running? Thank You