OK, let's start with the easy stuff.
You might want to take a look at the following article from HP. If you're having
major problems, you may have no option but to reinstall XP.
HP and Compaq Desktop PCs - Obtaining a Recovery CD or DVD set
http://h10025.www1.hp.com/ewfrf/wc/genericDocument?lc=en&cc=us&dlc=en&docname=bph07143
If you end up having to reinstall XP, take a look at this article.
HP and Compaq Desktop PCs - Performing an HP System Recovery in Windows XP
http://h10025.www1.hp.com/ewfrf/wc/genericDocument?lc=en&cc=us&dlc=en&docname=bph07145#bph07145_doc
That said, here are a few other suggestions.
First off, there's a command line version of XP's disk defragmenter. It can be
run by doing the following;
Go to Start and click on Run.
In the Open box, enter cmd.exe.
This should open a Command Prompt window.
At the prompt enter the following and hit the Enter key;
defrag.exe C: -v
You'll see an analysis report first and a final report when the program ends.
As for the problem you're having with mmc.exe, I would call the following a
solution of last resort. Microsoft states that this file will work with Windows
XP. Scroll down to the Windows XP section. The file to download would be the x86
version.
MMC 3.0 update is available for Windows Server 2003 and for Windows XP
http://support.microsoft.com/kb/907265
Finally, having problems with Windows programs not working correctly and System
Restore taking hours to complete could be a sign of problems with your XP
installation or the hard drive on your system. The first thing to suspect when
multiple problems develop is some sort of malware. I'm not saying that's the
source of your problems but it's a possibility.
Good luck
Nepatsfan