what's preventing it from crossing over to the other partition

Well short answer: nothing.
There are many forms of malware and only a small percentage will infect other drives, the others usually go straight for your system files. In order for a virus to survive on your PC it has to either inject itself into some executable or find a way for itself to be ran at system startup (which involves modifying certain parts of your OS). So if you kept the OS on a separate partition, chances are that you'll be able to make all your problems go away by simply formating that partition and re-installing Windows (which means that everything, including the virus and modified system files will be replaced with new uninfected fresh ones). Once you do that, even if the virus did infect some files on your other partitions, it will remain dormant because it won't have a way to start itself.

So let's say you do get infected, you notice that your PC is slowed down, you're getting weird errors or whatever and your anti-virus software isn't helping. You'll want to use your Windows installation CD to format your OS partition (usually C:\). Everything on that partition will be deleted but that won't be a problem because you were smart and kept all the important stuff on another partition (for example D:\).

After you fire up your freshly installed OS, make sure you do not touch anything on the other partitions so that if by some chance the virus did infect any of those files, it won't have a chance to start itself again.

The next thing you want to do is to get a fresh copy of some anti-virus software and do a full scan of other partitions that may have been infected. Once the scan is complete and you're sure nothing else is infected you're finally in the clear.

I hope now you see why this approach might save you from a lot of pain, and believe me there are other things beside malware that may ruin your day if you don't take my advice