Pro Tools 7.4 requires a specific security update to be installed on your version of Windows XP in order to be installed. If your computer is missing this update you have two options.
1. Allow your computer to update using the Automatic Update feature but only allow updates dated before May 2008 to be installed. This will give you a fully updated SP2 without getting SP3.
2. Download the missing security update (KB926255) from microsoft at the link below and install it, then intstall Pro Tools.
Microsoft Secuity Update KB926255
This will only install the missing update from Sp2.
Sweetwater Tech Support has seen many customers having problems with third-party plug-ins and certain features of Pro Tools after upgrading to Windows XP Service Pack 3. Even though Digidesign maintains that Sp3 is fully compatible, we advise NOT upgrading to SP3 as you may experience problems with Pro Tools as a resault.
If you have upgrade to SP3, simply uninstall it and choose one of the options above.