I do agree that as soon as more people begin to use linux, more security holes will be found and more viruses will be written for linux.
As boristsr said, you can't install or run anything that requires admin priveleges unless you are root, which most people don't (and shouldn't). In XP, you can't get anything done unless you have admin priveleges, which is why XP can be easily compromised.
I don't know much about Vista but I have heard that you will be prompted several times before anything can be run as admin. How much more secure will this make Vista; will it be on the same level as linux as far as preventing malware from owning your machine?