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Try breaking this problem into smaller pieces: Ping the server to see if DNS lookups are fast.
Write a simple static HTML page and request that to see if it's fast.
Try some simple database queries
The first step to fixing the problem is to find out what the actual problem is.
If the response is the same then look more towards the scripts themselves of your network hardware and general network performance (move large files directly between machines, check the speed - something may be wrong). Try explicit port routing for port 80 - it may be that your router isn't doing a good job - or is spending a long time figuring out what to do with packets.
If you find that the linux live CD does a lot better, then look at your windows setup - check for software which is monitoring or scanning internet communications - things like software firewalls in particular can slow things.
Check your memory utilisation, make sure there's plenty going spare. See what happens to your CPU during heavy loads on the webserver - does it reach anywhere near strained?
Finally, if all else fails, reformat and install a linux distribution. In my experience they run Apache and MySQL a lot better than Windows XP ever does.