If you have been running that installation for 4yrs. straight that can explain quite a bit. The system registry often accumilates useless entries, the drive may be somewhat fragmented if you haven't defragged it recently, and the possible corruption of system files.
AVG is one of the better ones but nothing catches all everything. RegClean free from Microsoft and RegCleaner freeware as well can find a number of lnvalid entries in the system registry itself. Besides Windows other installed programs can develop problems over a period of time. Besides running the msconfig utilty to reduce non essential startups a clean partition with a fresh installation of Windows may be needed. Four years without a fresh install of Windows to work with?
The current memory installed and a need for a bios update are two other things to consider with any older system besides the battery on the board itself. Does your friend have an installation disk? Besides anything lingering from the infection a run of the system file checker to replace/repair any damaged or missing system files may help. You simply type in "sfc /scannow" at the Run prompt and press enter to start that. Usually it will ask for an installation cd for verifying system files.