Ram Trouble Please Help!!!!!!!!!!!

[xxamdxx]

I bought half gig of ram today, its compatable with ddr 400 ddr 333 and ddr 266, and my computer can support pc2100 which is ddr 266, my computer is reading that i have more ram but an error screen comes up and it restarts my computer. Whats going on????¿¿¿¿

 

[LaptopExtreme]

What RAM are you mixing it with? Also some motherboards dont require mhz alone. Other factors can be non-ecc, must be paired the same, or speed.