Sorry for the lack of response. None of these programs were satisfactory, so I just reformatted. The virus appeared to be gone, but I had the same virus last year, and it's pretty vicious. It retrieves all logins from your FTP client, and uploads itself to every site that you admin. I couldn't take any chances, so I had to start from scratch.
Thanks for all of the responses.
One follow-up question. When I run Avast, it finds one infected file on my D drive in a hidden folder (System Volume Information), but when it tries to delete or move to chest, it says the file can't be found in red text. I can't access this folder at all manually, so I can't see whether it exists or not, but Avast says the mystery file is infected. Ideas?