Windows XP SP3 with 2GB ram on a single core or early dual core processor is at End Of Life for both the Operating System and the hardware it is running on.
The last Windows XP patches (November) had a glitch that resurrected Windows XP's Windows Update WUAUCLT.EXE svchost wrapper 99% processor usage demon.
This has been an ongoing problem that Microsoft will not be bothered to fix, September Windows XP patches randomly brought the above issue back, October saw an update to Internet Explorer to fix it, a larger quantity began suffering this time around in November.
Basically, when WUAUCLT.EXE (Windows Automatic Update) fires off to check for available updates, it can trigger the svchost.exe wrapper 99% processor usage. You can actually trigger it simply by trying to manually run Windows Update. You can see this in operation by pulling up Task Manager and sorting processes by cpu usage. Look for svchost.exe at 99% and if that's the case, none of the other recommendations here are worth diddly.
Two things to consider, ban your antivirus from scanning Windows Automatic Update related files and folder as per KB822158 and try reapplying the October Internet Explorer MS13-080/KB 2879017 patch manually.
Next Internet Explorer Rollup to fix it was KB2898785 dated for Dec 10, 2013. The SVCHOST 100% CPU issue marches on.