Great topic, arunpawar!
Here's some of the biggest myths I've come across are:
(1) Vista doesn't run some games. I'm sure it doesn't, but it runs all my old ones quite well. (I'm not smart enough to figure out the new stuff.)
(2) Linux is hard to learn. Not really. If you setup any distro and leave it alone, the learning curve is microscopic, since so many apps are bundled with each distro. They already come loaded with OpenOffice, a text editor (or 3), multimedia apps, email/browser/PIM, archive app, up to 180 little games! and so on.
(3) Using third-party ink in your printer voids the warranty. Nah.
(4) You have to partition a large hard drive and/or defrag it often to get the best performance. Partitioning is a good idea to protect your data from the OS and system files, but defragging for performance improvement is really overrated on Windows, and never required on a GNU/Linux machine.
(5) Macs are safe from viruses. Nothing is, not even Linux. But OS X and Linux both have tens of thousands of fewer verified virus signatures than Windows. That said, Vista is certainly harder to damage than pre-2007 XP.