have you tried your computer without the new RAM just to confirm that this IS your problem?
you say it is the same specification... same speed aswell i presume? (PC2700)
have you checked for viruses/spyware?
If your computer works normally without the new memory in, try with the new memory on its own, and then with one 256mb, if the problem re-occurs, try the other 256mb
If the problem occurs when ONLY the new memory is installed, this will probably be your problem (either incompatibillity, a faulty slot or damaged RAM)
also try this
test, un-zip it to a floppy, then boot your computer from the floppy... it will report any errors it finds (do this with the new memory installed)