Slow Startup

Hi,
My Computer:_ Intel 2.8,2gb DDR, running XP+SP2
I am fast thinking that I must have a virus of some kind. My computer now takes over 7 minutes to load and my Skype phone(which I rely on a lot) is now garbled and cannot be used and the computer is now very sluggish, as if my AV was running. I have trimmed my startup menu which made no difference.

The light on the HDD during boot is glowing bright all of the time. I have also defragged my C drive again this made no difference.
I have downloaded Hijackthis and have a file which I can cut and paste if there is anyone who can help

chkdsk checked my C drive and on the 3rd section, security I think it was, it started to repair a file, then startup threw up the message isass.exe corrupt press OK to continue, I did and the computer just hung, I had to restore to just before chkdsk. I,ve googled isass.exe and it appears to be a virus, this exe file is also present in my HJT file.

Desperate for help as I use my computer a lot.

Regards,