Bios detects ram, XP Doesnt.

I have just installed 2x512mb Kensington pc2100 DDR ram sticks in a friends computer. The previous ram: a 1x128mb pc2100 DDR stick. The computer ran very slowly, and I offered to upgrade if they bought the ram.
The BIOS detects the new 1gb, as does Piriform's system info tool, Speccy, but the system properties windows does not and the computer is just as slow as it was before. Any ideas?

RECAP:
Old ram - 1x128mb pc2100 DDR
New ram - 2x512mb pc2100 DDR
Current ram - Both

Issue - new ram not being used.

Thanks for all answers,
Jeff