Look in the user manual for your monitor, it will state a "preferred resolution" or "optimized reolution" or something like that. This is the resolution you MUST run Windows at to make it look anywhere near decent.
My 19" wide runs at 1680x1050.
Next, (and this is where your problem probably lies) you have to make sure your PC can deliver this resolution. Some older "onboard graphics" cannot deliver the higher resolutions.
The best solution is to buy a cheap graphics card (I am assuming by your technical level (no offence) you do not need a high end graphics card).
All modern cards will deliver the resolution you need don't pay more than �30 for one. You then have to get it fitted, you should be able to do it yourself by following the instructions.
If you don't want to spend the money it may be possible to configure your PC to allocate more RAM to the graphics to allow higher resolutions, this will slow your PC down though, how much depends on your PC spec. How to do this is another conversation but you may find it in the the user manual for your motherboard if you have it.
GL.