PC always at 80-100% cpu?

[Nahum]

You don't mention what sort of hardware you have. If your system is low on RAM (especially if it only has 2GB or less) or hard drive space (less than 10% or 2× your RAM), it is struggling to simply set up the desktop.

It could also just be doing regular maintenance such as checking for updates, indexing your files for search, or running antivirus.

If Task Manager isn't giving you enough clues, try using Resource Monitor (Start > resmon.exe or go to Performance tab in Task Manager and press button) to see if a process is taking up CPU cycles or keeping the hard drive busy.

If you do suspect a virus, try running scans in Safe Mode.