yes, you might not have to format. Try googeling the virus name (If you can) you will be amazed at the wealth of info out there and fixes. If you are comfortable on your PC, you may be able to fix it in the registry...but only go there if you HAVE to...and if you are on Vista I am probably giving you dated advice.
Other than that, pray it isn't a damn Zlob, and try and back up anything important, because there is a good chance you will have to format.
Also, get a new anti-virus and tell your sister to stop looking at shady pr0n site or whatever :P