hidden folder+anti virus+usb
Try the AVG, is free and yet powerful.
Try check any suspicious file especially autorun file in ur memory stick, u can see the file by select the tool -> folder option -> vew -> show hidden files and folders. By right u shudnt have this kind of file and check if u have the hidden folder with long folder name (most of the char is in numberic). Delete this kind of file because most ot the time it is virus.
Also, if ur comp is infect by virus, there is always some common step to remove the virus
1) Remove all the temporary file, folder and cookie inside ur IE
2) update ur antivirus database
3)restart the comp in safe mode (by pressing f8, f2, f1 or delete which depend on ur comp)
4) do a fully virus scan, this may take long time depend on ur harddisk size.
If u know what the viruses name, it is even better, remember google is always ur best friend, try online search for the viruses description and and how to remove it.
*one more thing, u can always do a system restore first to see whether it solve ur problem or not, however it is recommended to do a system full scan after all