So, this is 99.9% not related to avast! You have major system files issue. Your Windows installation is most likely corrupt and it needs to be repaired.
You never mentioned what version of Windows you have. All possible info would be greatly appreciated.
But, before you do that, go to free online antivirus scanner called Housecall, just to be sure that your PC is not infected with some nasty virus. Here is a link for Housecall:
http://housecall.antivirus.com/housecall/start_frame.aspChoose country you reside in, then wait for program to update some definitions, and then select what areas of your system to be scanned. Believe me, you want to scann whole hard drive, or even all hard drives that you have installed. Click on SCAN button and let it finish with system checking.
Also, you may try to scan your system of spyware and adware. Use freeware programs like Ad-Aware and Spybot - Search and Destroy.