Fernando,
The message you are getting regarding the lsass.exe appears to be related to the lsass virus that attacks unpatched versions of XP. I would suggest downloading the network version of SP2 and burning it to a CD. Do a fresh install of XP (use the XP CD to format the drive) and then install the service pack BEFORE you connect the computer to the internet. You should also install anti-virus programs before connecting to the internet.
If you feel a little adventurous (and you have a CD burner) you might try creating a slipstreamed installation of the XP CD with SP2 already installed. This can be easily done with a program called
Autostreamer or you can do a google for a tutorial to do it yourself (a little harder).
Good Luck
-Erin