I have 6 PCs one each running on 98 and 2000 and four on XP. In comparison, 98(rev B) is excellent and generally very stable but lacks some critical facilities e.g. the ability to view picture thumbnails. That said, the machine that it's on is in regular use as a word processor and does a grand job. 2000 is brilliant. Never falls over, has the ability to show thumbnails (albeit only one file at a time) and is not too hungry for resources. Let's pass over ME, that was pathetic and had more bugs than the natural history museum. The one machine that originally had ME was upgraded to XP (without any problems).
XP SP2 is, in my opinion, the Microsoft operating system most fit for purpose to date. This is written 6 months after the release of Vista! Why? Well, in my experience XP is stable and capable of running the vast majority of software without crashing. It's not perfect but it does 99% of everything most people will ever need.
Mac OS - great but you have to buy an expensive Mac to go with it, so limited range and a WHITE KEYBOARD would last about 30 seconds with me and it's £50 extra for a black one!. Linux - now getting worth a look but still doesn't support much of my legacy software and a touch geeky. Vista - don't get me started here, just read all the reviews with less than 3 stars.
The good points about XP: stable, runs almost everything, only one legacy program I have won't run with it but that's not a major issue. Decent networking, reasonable number of automated task wizards, lots of built in drivers, good support and now well sorted (post SP2). Clear and easy to use interface easily customised to suit most users. Can handle both business use and most media applications.
The bad points: You can argue security and I would agree, but a decent 3rd party firewall & virus checker should see you ok. Otherwise, not much to grump about. Downside is that it's limited to 2GB of RAM.
Oh, I've now also had to remove Vista from no less than 3 machines and install XP Pro at the owners' request. Does that tell you something?
My intention is to hang on to XP Pro as long as I can and hopefully skip the Vista bloatware until something better comes along.