When you buy a new computer you must put your music onto the new computer by copying it either from the original computer or the backups of the computer that you were making, or downloading it from the original source online, or re-ripping it from the original CDs.
iTunes Match is one way to download from the cloud, although it is not the most effective. If you did not subscribe to iTunes Match beforehand then the service cannot help you now. Did you subscribe to iTM before the old PC "got a virus?"
Additionally, this Apple KB articles will help you copy your data:
http://support.apple.com/kb/HT1408
http://support.apple.com/kb/HT2519
Now that you have a Mac, and assuming you have not been backing up previously, it is imperative that you start using Time Machinge (or the backup software of your choice) to backup your personal data: http://support.apple.com/kb/HT1427.