This is part of the point that everyone seems to be missing. Not only does the OS get upgraded, but the hardware does too. I bet when win2k was released it was moderately fast on moderate hardware. Windows XP did the same when it came out.
When you try to install Vista on your XP machine with 512mb to 1gb of memory and a 1.5-2ghz processor then yeah, its going to be slower.
I have Windows 7 installed on both my laptop and my desktop, and I noticed a performance increase on both. XP -> Vista had a larger performance gap, but once hardware caught up (and SP1 fixed a few things) the OS ran fairly smoothly.
I work for an IT department and if I remember correctly, when XP came out, we didn't upgrade for a long time and mostly had windows 98 or 2000 because XP ran slower.
Stop trying to use an up to date OS with way out of date hardware and you may notice that it isn't so slow after all.
(No, I'm not a Microsoft fanboy, but I'm sick of people complaining and expecting the latest software to run on their old school hardware)