From the sounds of what you have here, no matter how many of the above mentioned tools you use.... there is NO way to know if you are running a uninfected machine even after successfully running the above mentioned programs.
A small variant in the rootkit virus will leave traces of the rootkit on your machine and can fundamentally alter the OS in ways not seen by any of the above mentioned programs.
I highly suggest you go about reformatting your hard drive, or for that matter, go out and buy a new physical hard drive and reinstall WinXP onto that new drive.
ALSO AS NOTED ABOVE, COMBOFIX.ORG IS A MALWARE WEBSITE POSING AS A LEGITIMATE ANTI-MALWARE SITE.
ONLY DOWNLOAD THE COMBOFIX PROGRAM FROM www.bleepingcomputer.com !!!
True, it must be from BleepingComputer, Witches have found other places.
Better to download the disk utility from your HDD's maker and do a full low-level format. Lots cheaper than a new HDD and wipes everything off the corrupted drive.