Old Compaq Pc From 2000 Windows Question

Three issues. One - Tried to upgrade - to 1 GB Ram--did not work. Not even when I tried to plalce an additional 256 MB. It came with 3 slots but only 512 MB split among a  two 256 MB chips. Tried two 512s would not work.  TRied 1 512, two 256, then three 256s.  Two- Never had a problem with authentication before. Software was validated sometime ago.  Now Windows says my software has been counterfeited or  hijacked. Probably hijacked.  Three- Won't let me use my Norton Security Suite, see my icons, or get into saefly remove ny flash drives. Can someone have hijacked and gotten product key and used on their system?   ( Have Windows XP home on this PC. Have Windows Xp Pro on Dell laptop I purchased online just recived Dec. 23, 2010. So decided to give Pc to my son, by the laptop not working so returned to use PC until I troubleshoot to  fix laptop. It was refurbished and I pray that it does not end up with same problem as the PC when I update).   Compaq PC is for my son now--I gave it to him, because I thought I would only be using my laptop. However, 1-19-11 had a Symantec tech  assist by remote and since then the Problem  with Genuine appeared.  This Compaq PC installed with Windows since purchase from an independent seller.  I don't have the desire nor funds to purchase yet another copy of Windows. I tried, did not like Vista and I don't need Windows 7.  This is wrong. We spend all this money for what? To be told our copy is valid, then not valid  later.