Sorry that didn't work, it may be the boot sector instead of the MBR.
I don't think anyone can make a determination about if the system is infected or not. From what I have seen so far I would say no because the problem you had loosing the partition seems like more of an accident then something deliberate because only partial information defining the partition was lost. The MBR, partition table, and boot sector are all in the first few sectors on the hard drive and whatever caused a problem there could have been caused by several things.
If you have a XP installation disk you can write a new boot sector by entering the rcovery console and typing "FixBoot" and "FixMBR" again.
Do you have a XP installation disk?
Dave