The reason Win98 is more 'secure' now is that no one writes viruses for it any more, and those that did exist were patched out shortly before MS dropped support for it. However, once one of those viruses actually gets into a Win98 system, you're totally screwed. Win98's memory protection is practically non-existent, so it doesn't take much for the virus to work its way into the kernel, even if you're not logged in as Administrator.
ZB: You've evidently had more luck with Win98 than me. My system used to last about six months on average, until the Registry started doing strange things. I suspected a hardware problem but tests didn't show anything up.
On that note, I've recently discovered that Windows is capable of randomly losing files. The systems where I work are missing some directories that haven't been deleted by scripts or users. These machines haven't even been logged into in over a week. Of course, they are Virtual Servers, and the relevant drivers may be doing something odd.