I would say that thye motherboard can't run the 1800+ because that is
quite a big leap from a 900MHz Duron.
ID the motherboard and visit it's manufacturer's site for a manual that will tell you which processors the board can run.
XP 1800+
QuantiSpeed™ architecture
266MHz front-side bus
462-pin Socket A interface
128K on-chip L1 cache
256K exclusive on-chip L2 cache
Fully pipelined superscalar floating point engine
3DNow!™ Professional technology and superscalar MMX™ technology
0.18-micron manufacturing process
1.75V core voltage
37.5 million transistors
128mm^2 die size
"Besides the hardware changes, you've obviously noticed that AMD has slightly changed the name of its flagship processors. The new Palomino processors are denoted by the new "XP" designation. Unlike previous 7th generation processors, the Athlon XP processors are branded based on their performance relative to conventional Athlon processors. So in the case of the Athlon XP 1800+ we're testing today, the chip offers similar performance as AMD's previous Athlon product at 1.8GHz even though it's only running at 1.53GHz."
Here are the specs for the Duron 1000
200MHz front-side bus
462-pin Socket A interface
Large 128K on-chip L1 cache
High-performance 64K exclusive on-chip L2 cache with hardware data prefetch
Fully pipelined superscalar floating point engine
3DNow!™ Professional technology and superscalar MMX™ technology for great multimedia performance
0.18-micron manufacturing process
1.75V core voltage
25.18 million transistors
106mm^2 die size
The FSB is 200MHz for the Duron and 266MHz for the XP 1800+, so if you can't set the FSB to 266 on your board, you won't be able to run the 1800+ at full speed even if you got it to work at an underclocked speed.