I think the issue OP is facing is Hardware related - read somewhere that one guy had this issue due to a bent pin on mobo cpu socket though that guy fixed it 
@ OP - follow these :
1. if possible check your system with one mem modules at a time and when using one 2GB mem module only see if win 7 shows you 2Gb ram under sys info . Do the same with other mem module.
2. Check each ram slot by installing one 2GB mem module only - repeat the test with another mem module.
3. Physically inspect the mobo cpu socket by properly removing the cpu - if everything is OK then remount the cpu and heatsink properly
4. Check both mem modules by running memtset app.
@ OP - follow these :
1. if possible check your system with one mem modules at a time and when using one 2GB mem module only see if win 7 shows you 2Gb ram under sys info . Do the same with other mem module.
2. Check each ram slot by installing one 2GB mem module only - repeat the test with another mem module.
3. Physically inspect the mobo cpu socket by properly removing the cpu - if everything is OK then remount the cpu and heatsink properly
4. Check both mem modules by running memtset app.