uninstall unneeded apps (and use sysinternals autoruns, and hijack this)
run ccleaner
use diskview or treesize to hunt down space wasters
ghost remaining for safety
run manufactures test program...and also look at SMART status indicators
remove disk to known clean machine,scan with at least a half dozen antivirus and antispyware tools
burn cleaned data to dvd
reinstall and attempt booting in original machine
reinstall and correct damaged / corrupted files
reinstall protection software
redo all updates
retest function of all used features
burn working image