The system requirements for Vista according to Microsoft are:
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1 GHz 32-bit (x86) or 64-bit (x64) processor
1 GB of system memory
40 GB hard drive with at least 15 GB of available space
Support for DirectX 9 graphics with:
WDDM Driver
128 MB of graphics memory (minimum)
Pixel Shader 2.0 in hardware
32 bits per pixel
DVD-ROM drive
Audio Output
Internet access (fees may apply)
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Now, your processer seems OK. Your hard drive is pushing it, but should be OK. Your RAM is only a quarter of the minimum requirement, so Vista won't even be able to start up.
Even if you did upgrade your system to 1GB RAM, it's still going to run really slowly.
So, my recommendation is to either buy a new PC (I'd recommend Dell - They have great customer support, easy ordering etc.) or reinstall XP on your machine.
Hope that helps!
-James