Based on what you’ve said, I’d monitor the system for a bit, as you say it seems to have recovered its’ former speed after a reboot. You could try a run of the free version of Hitman Pro and see if it finds anything (I doubt it). But be careful of what you do if it does find something, always quarantine the suspicious file just in case you need it and it is found clean later.

If you feel you need the help of a certified malware expert to ensure nothing is amiss, then go here: http://forum.avast.com/index.php?topic=53253.0

Download and run these three programs: Malwarebytes, OTL, and aswMBR.exe. Attach all logs in your next reply should you decide you do need this help. You can attach the Malwarebytes log since you already have it.