you may want to update to SP3, it'll plug some security hole and give better compatibility for hardware and security certificates, and may even resolve your grayed out advanced options, in XP MSCONFIG you need to select BOOT.INI and ADVANCED OPTIONS and the tick "/NUMPROC=" and then select the number of CPU's you want to use.
I'm trying the find a reg frag to do this but it's proving abit illusive, even with procmon, but as soon as I find it I'll post it up
I can definately recommend a few other tips as well for XP, bootvis.exe (MS) will drastically reduce your boot times, as it loads all your drivers side by side and not one at a time.
Auslogic defrag has the option to Optimize your files when defragging, i.e. it'll put the most used files on the fastest part of the disc plater, do this one before bootvis though.
black vipers reg tweaks are a must for XP, just to squeeze every bit of performance out of your rig. (I know BV again).
and if you can find get your hands on XP performance edition (you already have a license for XP so your not breaking the law here), it's tweaked for performance, and has really low over heads, giving more for rendering