New ddr1 400mhz, pc3200 memory crashing computer?

I've installed one giga bite of new memory but the problem is when the computer boots up everything loads, then when you go to open a program the computer automatically reboots. Is this a problem with the new memory?. The computer recognises the new memory and I have tried it in the other free bays but to no avail. I've taken the old memory out as this is a different size but eventually put it back in and removed the one gig. Any advice would be great, Thanks P.S. win xp, 120gb hard drive, 2500gb athlon processor.