Well you never said what OPS you have firstly.
Probable causes, 1) Motherboard don't accept more than a certain amount of r.a.m.
2)OPS don't support more than 500 mbytes of r.a.m. (though win updates may have a fix to sort that if you tell him to run win update, or check knowledge base).
3)Motherboard especially award aka pheonix go manky after a period of time, my old socket 7 used to tell me it only sees 2 mbytes when I had 32 installed on win 95.
4)Could need a bios upgrade.
5)The r.a.m. may not be in the correct amounts per slot, like 5 slots with 64 in each rather than 256 +32, 128+128+64+128 etc.
6)Swap file set incorrectly set.
7)R.a.m might be faulty.
8)Some applications need a heap of r.a.m. to open like the one you mentioned, just disable some plugins, I posted a few weeks ago a utility what does this for you.
9)Internal hardware might be sucking up too much energy (Juice).
10)Out-of-date drivers.
OR A VIRUS IS SCREWING YOUR DAD'S SYSTEMS'S