Defragmenter and Spybot now work again. Spybot cleaned it up after leaving it to work for 10 or 20 mins. After that the defrag worked too. But Disk Cleanup still won't work. It gets stuck on 3 bars, with CPU usage at 100% and causing it to get pretty damn hot. When it is doing this, there is no HDD activity either, except the few occasional blinks.
Even if I close the Disk Cleaner, it still leaves the process running, with 100% usage. I have to close it manually via Ctrl+Alt+Del.
I also uninstalled Trillian, and that hasn't helped. Any ideas would be helpful, so I don't have to spend £35 for Microsoft Tech Support.