I just got done re-installing Windows XP Pro SP2 on my dad's comp. with intel 2.4Ghz and 1Gb ram. and re-installed Halo 1 for PC back onto the comp. now it runs like ****. he has a Nvidia Gefore FX5500 256 MB with the latest drivers from nvidia, the 169.21 and yea...runs like crap. it is very laggy, and sound is not synched with the video and I chose Vsynch in the options with all other options on low

try uninstalling the game and reinstalling it. make sure that the registry doesn't even retain the old halo 1 entry.

reinstall it completely, and see if that does anything to improve it. the thing here is, it's safe to rule out viruses and drivers. that doesn't leave much except hardware and the actual game.