Darn near impossible (or at least impractical). The proportion of computer users who dual-boot Windows/Linux is so small that it's not worth anyone's time to write the code required to access a Linux partition from Vista and corrupt it. And it would have to be done intentionally; I'm sure it's safe to say that Windows will not have a major bug in the filesystem code that wreaks havoc on the HD with complete disregard for partition boundaries.
So what I'm saying, in less-than-eloquent terms, is not to worry about it. At all. (Unless you install a driver to access your Linux partition(s) from Windows, then, you might want to worry just a little bit.)
1This is not fully related to your question, but I know XP has always been selfish when being installed it would not play nice with other OS's. You may need to "fix" grub afterward installing Vista so Ubuntu will boot again. – Jon – 2009-03-16T15:10:50.617