It could theoretically be the XP processor. It's not as good as the P4 in some areas. System requirements are here: http://www.apple.com/quicktime/guide/hd/recommendations.html
Other areas to check for might include OS or software setup:
Stuff messed up on the OS
Drivers that are wrong or causing problems (need to get an update)
Bad quicktime install
Other programs/processes running in the background, from legitimate items, to items that you don't know about, to just plain old malware and spyware.