Hi I hope someone can help me...

I have a Athlon XP 1800+ CPU (266MHZ) with 512MB (2x256MB) PC2100 DDR Memory and wanted to upgrade to 2GB of RAM with (2x1GB) PC2100 DDR Memory so the CAD software I am using will run better.
I bought 2 sticks of Major 184 Pin - SDRAM - 2.5V - DDR - PC2100 - Non- ECC - 1GB module and inserted them in the alotted slots but when I turned the computer back on screen error said:
Windows NT has found only 0K of low memory. 512k of low memory is required to run Windows NT.

I went in to the BIOS and it stated I had only 1KB of Memory and it could not be changed so what should I do?

Hello howler, welcome to PCHF
Try reseating both of the memory modules. What brand are the memory mocules? (Corsair, OCZ, Mushkin etc)

Howdy howler and welcome to PCHF

Can you tell us what motherboard you have

Your motherboard may not be able to handle that much, you can check by using Crucial.com's Memory scanner by clicking the link below:
Memory upgrades from Crucial.com - About Crucial:Media Toolbox

Howler-- I have a suggestion-- you have your hardware listed--
but none of us can see the info without your permission.
In UserCP-- where you listed your gear-- there is a check-box to "allow others to see my specs".
Place a check there and yours will show like mine do.
Saves a lot of time.