Avast detects the blaster and sasser worm without problems, also the bored/guru infection is detected by Avast without problems. So why it doesn't on your moms comp (which should have been patched in the first place a long time ago!) is a little riddle we have to solve.
Both with blaster and sasser there was a patch released prior to the release of the worm, so if a system still gets infected it is the users own fault imo.
Hijackt this doesn't find any virus, spyware, adware, malware. It just displays a list of items from which YOU have to find out what they are and what should be kept/removed.
Some infections disable the AV software. Not saying this happened here, but it could be.
Please answer the following questions, lets see if we can find out what went wrong.
1) What version of Avast?
2) What version of the vps?
3) What os?
4) What settings of Avast?
In the mean time I suggest you click on the link in my signature and follow the instructions on that page to clean the system.