Well after lots of wasted time I finally found what was causing my Apache service to spike to 100% CPU.
By the way, I'm running Windows XP Pro x64 but it probably isn't relevant to this issue.
It wasn't any of the following:
Xampp Control Panel
Adding Win32DisableAcceptEx in httpd.conf stopped the Apache service loading.
Reinstalling didn't help
I also tried WAMP 1.7.0 & 1.7.2 with the same outcome.
I finally realized what it was when looking through my add/remove programs.
A windows port explorer which I had only installed a few weeks back.
Not sure why this happened but removing it fixed the spike.
So, if you have any port scanning/exploring tools or other network tools you should remove them if you are having similar problems and you have tried the above Win32DisableAcceptEx fix.
Hope that helps someone.