XP is in short, the best system and the worst system EVER.
it's the DEP (Data Execution Prevention) software. Look up BartPE. It's what I like to use.
here's how to do it:
1: when you are done editing the file, save it to your base folder (C:)
2: after you get a BartPE (or some other 'right off the CD' operating system) CD, put it in the drive, and restart the system. when it gets to the boot screen (no, it won't be edited yet, obviously.) hit F12. this will bring up a different screen.
3: select option '5: load from CDROM' (or something like that)
4: let it do its thing, it will take a little while for it to run, cause it's working off of your CD drive
5: if you are using BartPE, click 'GO' and start the file browser/explorer program
6: locate and copy your edited file in C: (yeah, it takes a while, that's why you put it there.
7: go to the destination folder (for boot, you are in Windows/system32 i think) delete the original file, and check for and delete any backup files that might be with it (.bak, .whatever anything that has the same filename as the registry file you are editing)
8: paste your edited file in the destination folder, and select the reboot option from 'GO'
9: remove the CD after your computer begins to reboot again, and see if your efforts have paid off.
Hope that helps! I remember when i first started hacking M$... it's frusturating at first. you have to 'trick' it into doing what you want it to.