Update for Windows Vista (Read original article below first.)
Windows Vista does not have a boot.ini and uses a new form of boot loader. The official way to edit the boot settings is to use the included "bcdedit.exe" command line tool. However, using this tool is not easy and requires an advanced knowledge about using the Command Prompt. A wrong command could create serious problems and even render your computer unbootable.
NeoSmart Technologies has a tool called EasyBCD which allows you to make changes to the boot loader of a Vista system, just as easily as you would edit boot.ini in an XP system. It provides a GUI front to the bcdedit.exe command line tool, making it very easy to enable or disable boot options. You can even install EasyBCD in Windows XP, if you have your system configured to dual-boot with Vista.
To disable PAE on Windows Vista, download, install and run EasyBCD. In the 'Advanced Options' page of EasyBCD, Change "DEP/NoExecute" to 'Always Off,' uncheck "Enable PAE (Physical Address Extension)" and click "Apply Settings."
Original Article for Windows XP
Have you noticed if you have a line that says "Physical Address Extension" in your System Properties or System Information? System Properties can be seen by right-clicking My Computer and clicking Properties.
PAE is a pre-requisite for Data Execution Prevention. By doing some research, I have found out that you only require PAE if you meet at least one of the two conditions below. You can safely disable PAE if you don't meet them.
1) You have more than (but not equal to) 4 GB of RAM. (I only have 1 GB, just recently upgraded).
2) You want to use DEP capabilities of your processor. (I don't really think DEP is useful at all, I feel it's more harmful than useful as it can crash any program that it suspects. I'd rather trust a full-fledged anti-virus program than DEP.)
In other cases, it's best to turn PAE off as it will increase processing performance and hard drive access speeds.
Many people have asked how to turn off PAE, but not many have found definite solutions. It is because of the confusion of the /PAE attribute in boot.ini file present in the root of the C drive (or wherever your Windows is installed). This option is most likely not present in your boot.ini file. Turning PAE off is very simple though. You just need to change the /NoExecute=OptIn or /NoExecute=OptOut option to /NoExecute=AlwaysOff and save boot.ini file. This will disable DEP, and in turn, it will also disable PAE because if DEP is not enabled, PAE is not required anymore.
Restart your computer and check if PAE is still enabled. Mine was immediately disabled, but if for some strange reason (provided you have 4GB or below of RAM) it is still enabled, you can also try adding the option /NoPAE to the same line as the /NoExecute option. But be warned that this forces PAE to be disabled, even when Windows determines PAE is required for things other than DEP.