Make sure you installed your new RAM properly. Make sure the RAM sticks are fully seated in their slots.
Run a memory tester to make sure the RAM isn't faulty. ( http://www.memtest.org/#downiso you will need to burn it to a CD, or copy it to a floppy/thumb drive, and boot from it to run memtest) If it says there's even one error, that's one too many. It's either not installed properly, it isn't compatible with your system, or it's defective (least likely, but not impossible).
And next time, make sure to give as much information about your system and the components you've bought in your topic post as you can. Without knowing your specific situation we can only give you general solutions that may or may not help you. (You will also receive less replies because you make it harder for people to help you.)
Well if you have 4, 1 gig sticks then there is a possible explanation: some MOBOs have a bit of trouble running stably with all four RAM slots filled. The fix:
1. Read you manual, it may specify settings for 4 sticks.
2. Be absolutely sure that your RAM is receiving mfg spec. voltage.
3. You may need to relax the timings, or even the RAM bus speed to get 4 sticks stable.
Or perhaps your new RAM is either bad or so far out of spec with your existing ram that they can't run well together.
Try the voltage first. Make sure it is getting whatever the sticker says, 2.0 or 2.2 etc. MOBOs default to 1.8 and don't usually set the voltage automatically to spec.
Then test each stick individually.
Make sure you RAM is reasonably matched. If one set wants 4,4,4,12 timings and the other wants 5,5,5,15 for example then you will have to set your BIOS to 5,5,5,15
If none of the above helps then try relaxing the timings as per the example above. In other words even if all of the RAM is speced to run at 4,4,4,12 take it up to the next slowest rate whicj would be 5,5,5,15 and also maybe take it from 1t to 2t, etc. You might just have to play around with the settings till you find the right ones.