The 2 main reasons that Service Pack installations fail are:
- malware
- outdated drivers
In particular, SP2 was a huge step forward for XP - and if you don't have SP2 compatible drivers there's no telling what'll happen when you try to install the Service Pack. This was a huge problem back when SP2 came out!
PC Manufacturer's are aware of this, and have ensured that the proper compatible drivers are uploaded to their website. FYI - here's a list of when Windows versions and Service Packs were released: http://www.carrona.org/winreldt.html SP2 was released on 6 Aug 2004 - so those drivers that were released around that time (or after it) are definitely those that you have to be really concerned with.