|You should not unless you know they are really unused.|
There is no loss to you to leave them compressed. In most cases you will never notice any issue unless you are using very large files.
If you really insist playing with it, I think you'd be better off with a clean install, add in a/v anti-malware firewall and latest service pack then use best practices to keep system working.
"Best Practices", Event viewer, host file, perfmon, antivirus, anti-spyware, Live CD's, backups, are in my top 10