If you get access violations or lockups in previously working applications, it may be the Heap Manager Change in Service Pack 4.
To verify it is a poorly written application:
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\Image File Execution Options
From the Edit menu, Add Key and enter the application.exe of the executable that experiences the problem, without a path. See the Game.exe sub-key for an example. Class may be left blank. Select the application.exe key and Add Value name DisableHeapLookaside as a type REG_SZ and set it to 1.
If the application.exe is a service, restart the computer.
Now run your application. If you get more or less errors, contact the vendor or search for your application in the Knowledge Base.