I've Got An Advent 3111 With A Micro Star 6579 Motherboard I Know It Takes Pc2100 Ram, So I Bought 2 X 1gb Ram Sticks From A Power Seller Of Ebay Who Assured Me It Was Compatible With My Computer. Am I Doing Something Wrong Or Have I Been Had?

is the only ram in your system what you baught from ebay?
if so does your computer boot properly?

if it does, click start and rt. click 'my computer' and click properties on the 'General' tab, it should have your computer info at the bottom, it should say 2GB RAM, if it says 1, your 2nd ram socket is probably disabled in the bios
1) enter the bios
2)go to System memory and press enter
3) make sure everything is enabled

(i will check these instructions now)

I pulled out the 512 stick and replaced it with the 2 1gb sticks but it wouldn't boot up. I put the 512 stick back in and left 1 of the 1gb in but it still wouldn't boot up. The new ram is high density ram but the seller assured me it would work with my computer. is there anything i'm doing wrong

just try putting 1x stick of ram in where your 512mb ram was, see if it will work then.

If the one stick of 1GB will work alone then density is probably the issue.

Density only comes into conflict when you max out a board.

Also verify the 2.5V

Support 2.5V DDR DIMM

I've Checked The Spec Of The Motherboard And It's States That It Can Take 2x 1gb Sticks Of Ram Pc 2100 2.5v Unbuffered Dimm. The Spec Of The Ram Itself Is As Required Except It's High Density. None Of The Sticks Will Work Whatever Combination I Do. Any Suggestions???

I can only educatedly guess that that RAM is not compatible with your computer.

I doubt that 2 sticks could be bad in themselves and if your old RAM works I doubt that there is any problem with the motherboard.

Beyond that I don't know what to suggest except try other RAM.

Sorry