virus, need help.

ive gotta problem. one of my computers has a real bad virus on it. its locked down my anti virus software and wont let me run a system restore on safe mode. When i first found it it shut down my computer when i tried running a scan and since then it has changed my desktop bkground. if anything i just need to know how to extrack some files on to a flashdrive and to be sure they are not corrupted. thanks.