The other day I was trying to install PHP and apache server on my Vista machine. As I went ahead it got a little bit complicated and some things didn't work out. Then had to search for stuff online and finally got hold of this article which works fine.

Installing Apache:

Uninstall any previous installations of Apache Web server (Start > Control Panel > Programs and Features).

Turn off your firewall (Control Panel).

Stop User Control Access(UAC). Do this by turning it off on your vista machine.

Get the most recent version of apache from http://httpd.apache.org/download.cgi and put it on your desktop. Rename it to apache.msi

Start > All Programs > Accessories

Right-Click “Command Prompt” and choose “Run as Administrator”

Manually remove directories containing previous apache installations (like C:\Program Files\Apache Software Foundation…)

Change to your desktop folder (At prompt type cd desktop)

Type “msiexec /i apache.msi” on the command prompt.

Run through the Apache installer. I’m running a development server, so I left the domain and computer name blank. Choose the default server on port 80 for all users option. Change the installation directory to c:\apache.

Reboot.

The little Apache feather won’t appear on the task bar under Vista with the present version of Apache (2.2.4). To remove the “error” box that says ‘the operation completed successfully” on startup, go to All Programs > Startup, and remove the Apache item there.
Browse to http://localhost. It should say “It works!” If it doesn’t, check your httpd.conf file by going to All Programs > Apache HTTP Server 2.2.x > Configure Apache Server > Test Configuration. Follow the directions for fixing the configuration file.
Turn your firewall and UAC back on.

Now you can install PHP the same way you did for apache. (Follow the same steps as you would do for Windows XP, just turn off the UAC).