To rescue your infected PC, you first make sure the machine is turned off, and then you turn it on again with the CD, DVD or USB installed. This boots the computer directly into Linux, completely avoiding Windows and its infection.

From there you can get some good antivirus software to battle the infection. ClamAV with its ClamTK interface are my favorite solution, and can be obtained from the Software Center, for instance. With such software in hand, you can then proceed to squash the nasty bug that crippled your machine in the first place.

Once that's done, you can boot back into Windows and get on with things.

2. Banking

Such is the poor security situation in Windows that experts now commonly from conducting online banking or other sensitive transactions over a Windows PC. Once again, Linux live media can help you out here.