internet/modem nightmare

i just built a machine for someone w/the following specs.
amd 2500 barton retail
msi k7n2 delta-l mobo
2-256mb corsair value select 3200 ram
ati 64mb 9100 (visiontek xtasy)
on-board sound
comp usa v92 modem
xp home sp1

after some overheating problems, the retail pad was bad, and i replaced it w/silver the machine was up and running ok. i delivered it to my aunt. set-up her earthlink and then downloaded the printer drivers. the machine seem to be working ok so i went home. the next day she called me back and said she was having problems w/the internet. it would only allow her to stay on for about 5 to 20 minutes. after she called earthlink the problem got worse.

i thought it was a bad modem or it was a problem w/the phone line/cable. the phone company checked the phone line, nope it was ok. we replaced the cable, same thing happened. i replaced the modem w/the same modem (comp usa v92) that i use. i un-installed the current modem, software etc. i rebooted and then it would not go into windows. it was in a continous rebot because of file "lsass.exe". safe mode would not work. xp cd repair could do nothing. i finally had no choice so i reformatted the hd. i also had already registered her xp w/ms. hopefully i won't have to much trouble w/them when we try and re-register it. anyway after it was back up and running i started to download her printer drivers from epson it was running smooth and then just stopped. i was still connected but ie just closed by itself for no reason. i decided to try and download the latest critical patches from ms figureing this could solve the problem. i waited for about 20 minutes and since it would take a few hours (18.6mb) and it seemed to be ok i went home. i got a call later saying after it downloaded 5mb it did it again. i've spent 8 hours on this machine on the last 2 sundays and i'm at my wits end. i've never had this much problem w/a machine before. i've built 10 machines over the last 3-4 years, both intel and amd and they are all running fine.

ok here's the point. she is going to take it to the local comp usa later this week but i would like to prevent her from having to spend the money. does anyone have any ideas of what could be causing the problem? the machine seems to running ok otherwise. if there is a particular critical patch of ms that i could download i think i can get to stay online long enough to download it, as long as it's not to big or i could download it at home and put it on a cd and install it then.

thanks in advance for your help.