Is that the 128mb or 256mb model there? Apparently you had a good day and likely had cooler room air or less of a load on the system in general. That would be one first glance.
The thing that comes to mind however especially with an older ATI AGP card is any antivirus program you have on. Sometimes shutting that down or the Catalyst Control Center was a rememedy for problems seen while gaming.
The best catalyst for gaming with the 9550 256mb run here on an old Socket A build turned out to the 5.11. Upto the 5.13 version was good while the 5.14 and 6.1-6.5 weren't. In fact those are still available as well as versions going back for 9x versions of Windows. The 5.11 and 5.11b are the last three at the bottom of the page at
http://downloads.guru3d.com/Videocards---ATI-Catalyst-XP-32-bit_c18.html
Generally the wrong version or too new a catalyst will see problems come right up much sooner then 2hrs. of being powered up. But too rule that out look at the version you have on now unless you simply used the original software disk when seeing a fresh copy of Windows go on.
On one old build here more then freeze ups occurred. I would be right in the middle of something and the system would suddenly restart itself or lock solid. That could be while running a game or video capture and then.... Then the cpu seemed to be cooked one day from erroneous cpu temps and the system failed to run with the fsb set for the model cpu run there.
When lowering the fsb from the 200mhz setting to 166mhz and seeing the cpu backclocked down it would run. But what was it? Well as it turned out the board was found to be the problem since it loved to eat batteries. After reassembling the old parts after a year sitting idle with a fresh battery of course the cpu ran like an XP3200+ again at the 200mhz setting.
Sorry to make this lengthy but that one problem seemed to explain a variety of similar things seen on the old build that you are running into there. Generally you would think a faulty dimm, temp problem, or driver issue of some type while a weak battery can also cause some odd problems to be seen. A weak battery effects the cmos information that then effects Windows in the end. Old board needing new battery? All too likely.