HI Zoe
Sound's like youre infected with a viruses/ Malware
download trend micro's DOS application here (2.1MB) and the two essential pattern files namely lptXXX and sspdaX_XX(X means latest versions)witch you can find at the same site
extract/install the DOS application and the files in a temp folder(all inthe same folder) in the root of local disk,now go into safe mode and run the DOS application
Ussually works Great
Cheers
Sittas