I want to clean viruses off my friend's computer but I don't want to be waiting for the antivirus program that I choose to run to take more than 30 minutes. What are the fastest freeware antivirus programs?:)

Nothing beats avira antivir.

It is fast, it updates everyday, and scans more processes that McAfee ever did.

I'm not sure about your 30 minute rule, mine scans over 258,000 processes, and takes about
33 minutes.

Try it, let me know what you think?

If you have your AV correctly set up to scan files on read and write then the need to do a complete system scan is negated.

Once the initial system scan is complete then read/write scanning will keep your system clean.

I think the time required really depends on the system being scanned. Although I've seen a lot of users satisfied with Avast as far as speed.
Hope this helps.