Its very possible it is a virus or worm. Avast is a good antivirus program. Have you tried running the "reboot av scan" or whatever its called?
in all honesty, if it is a virus/worm and its already done this, the damage isnt repairable (to my knowledge) besides an OS Reload/reformat. If you find a virus and it fixes it or deletes it, do an os reload. Otherwise, i think you're stuck with reformating your system.
Few things to note.
Did you notice these changes after downloading/installing a driver, warez, or using p2p? Any files your friends have sent you that didnt work or programs you have downloaded than never ran. Looked at pron lately
?
Are the problems happening one by one or all at once and is there a growing number of similar problems?
Also a thing to note... Are you using any custom theme such as the program Windblinds or something like the Windows XP conversion to Vista theme. These often cause these sort of problems.