It's a good idea to update the scanners virus definitions before running, but it's not always necessary to remove an infection. Boot discs are mainly for tough situations, when the computer isn't booting into windows, or is being effected at startup.
My advice is usually to use the boot discs to clear up as much infection as possible, and then to run through the malware guide in my signature. It's a thorough process.
I use Bartpe with some anti malware addins, super antispyware, spybot, malwarebytes and a few others. Hit up their webpage and follow the instructions. It is a pretty simple utility which creates a bootable CD and loads a windows pre shell. From there you can run your anti malware/antivirus utils (among many other nifty little tools built in) to your hearts delight.