There is a known problem of this happening after a ad-aware scan. They have information on it on thier web site. I will see if I can find it.
Start with thisForget that one for now...you can't boot at all right ? Not even in safe mode ?
Try this (I copied this so I would have to type it all out.)
Ever had a Windows XP machine that was in permanent reboot mode? Basically just before the computer goes into Windows, it starts over and does this cycle forever. It most cases it will also lock up when you try to go into safe mode. Well after years of replacing the harddrive of computers with this problem, I stumbled on a fix after trial and error.
This fix has worked on several Dell computers running Windows XP:
Boot to your Windows XP disk. When you get the option, go to "R" for "Repair"
You should now be a a dos prompt in the recovery console.
Type: chkdsk then hit enter. This will run checkdisk. If it does not run checkdisk (you will see the % ticker if it does), then run chkdsk /p to force the checkdisk
Now you should be at the dos prompt again. Type fixmbr
It will ask if you're sure, just say yes.
When done remove the XP CD and reboot into safe mode first. Select restart and let it reboot into normal Windows mode. Hopefully it comes up now and now you should be out of permanent reboot mode.
This MAY work... There is another option if it doesn't. (I'm still looking.)