How to get rid of computer virus msiexec.exe?

I have windows 7 and an alert keeps popping up that msiexec.exe is trying to make changes and asks if i want to allow it, and it also says that it originates from the hardrive. Everytime i say no, the same thing keeps popping up. I currently have avast free virus protection, and I'm running a scan.It's been 6 minutes and it's still 0% done. I just really want to know how to get rid of it, I'm also worried about it getting my debit card information since I made some online purchases.