Did you recently change something?
If it was hardware remove it for a test - unplug all your usb devices too.
Does it start OK in safe mode?
Do you have any virus protection and is it updated?
Is your copy of XP legit and do you have all the latest updates from MS?
Play any Sony produced cd-rom's in your machine (rootkit)?

Possible your HD is starting to fail or you have a conflicting hardware device during startup.
http://support.microsoft.com/default.as ... -us;315222Try booting with the bootlog enabled and post it.
The Boot Logging text is recorded in the Ntbtlog.txt file in the %SystemRoot% folder.