But the clues/detail are so slim in your post I can't begin.
The Pentium M is usually for laptops and may use another socket so you can check that out without my help.
Same for Intel's spec on power.
Finally you need to see if the Dell supports that CPU. You may have to ask them.
I have been running Win 7 on my old Dell 6000D 1.6 GHz CPU with 2 GB ram for a few years but I am bored with how slow it is. I found a good price for an Intel Pentium M Dotham 780 2.26 GHz CPU which would give me 40% increase.
Do I need to do anything other than plug it in?