I'm gonna presume your OS is XP Pro (as is mine)....I upgraded nearly 2 years ago from 1GB to 2GB and experienced identical problems
XP Pro is a pig with RAM it is so tempramental at times....I tried 7 (seven) different sticks before I could get them to accept each other
The only quick fix is to replace the lot with the same manufacturers brand I'm afraid...unless lady luck eventually smiles on you and finds two brands that 'like' each other
