Sorry for lateness of reply...
I have taken a note of what it says on the RAM currently in my PC and the one I bought to add.
PC:
1GB UNB PC2-4300 CL4
S/N: 230644112113
P/N: 78.01G66.9KB
New:
1GB PC2-5300
DDR2-667 CL=5-5-5-15
GX21GB5300LX
VOLTAGE: 1.8V
Right, even a man of my limited PC skills can see these are not a match - even though I only bought the new PC2-5300 after crucial.com told me that this is what was already installed on my PC.
I have done a quick Google search to see if you can mix these and saw this answer when someone had posed the question:
"1. If your motherboard is ONLY capable of using PC2-4300, it will just downclock any PC2-5300 that you put in there.
2. If you mix the two, BIOS will just synchronize their speed to that of the lowest which is the PC2-4300; even if board is compatible w/ PC2-5300."
I don't understand what it means, but, having tried the new stick again, my PC/motherboard clearly is not liking the 5300. Should simply purchasing and installing a stick of PC2-4300 be enough?
Cheers gents.