The t500 has three RAM slots. I have two 256 MB sticks and one 128 stick. I decided to remove the 128 MB stick. The computer boots up normally now, concluding that the problem definitely has to do with memory above 512 MB of RAM, even with the unofficial service pack.

Braca862 wrote:

When installing the unofficial servic pack 2.1, I selected the official updates and the performance tweak, nothing else.

Sound like an old version. I use 3.53 and with that version it's called "core updates" or something like that.