ram advice

[Maqua]

have had a second strip of ram put in to one of the computers where i work and it is now locking up in windows and sometimes asking for a system disk during the boot up, as the new strip is a different make and speed is this likely to be the source of the problem as the computer ppl the company use are trying to say it is the hard drive or maybe the mobo, as i don't know enough about this i would be gratfull for any advice