Hi qim
If you are totally unable access your computer to scan your computer, you will need to a Bootable Antivirus to scan and remove all malwares, Here is manual how to use one:
http://www.omidfarhang.com/computer/security/avira-rescuecd (You will need to Burn the disc using a clean computer)
And then return to windows and scan your computer using Hitman Pro:
How to use it: Download
Hitman Pro (or
Hitman Pro 64 Bit) to your desktop, Hold the Left Ctrl Key on your keyboard and double click on Hitman Pro to run it, keep Ctrl key holding until Hitman Pro screen appear. then click on next and let it scan your computer, let it remove the malwares it find, if it ask you for license active the 30 Days trial version. after removal, restart your computer.
if you have windows installation disc, after reboot your computer, insert the disc in drive, open 'Run' and type 'sfc /scannow' to let windows restore damaged or missing windows files from installation disc.
Now repair avast by going to control panel -> Add/remove programs (Program and Features in Vista/7) -> select avast, click change, in the opened windows scroll down to find repair, select it and follow setup to repair avast.