Resolution is saved by windows, not by Catalyst. You need to contact M$ support or simply reinstall the OS. You probably have a corrupted registy.
Wiping the display devices section of it might be enough, but only do it if you're familiar with it. Built-in system devices need to stay!!! You'll find the keys under HKEY_LOCAL_MACHINE\HARDWARE\DEVICEMAP\VIDEO. But if you didn't know that, don't touch it, call M$ support.
CPU: AMD Phenom II X4 810 @ 3120MHz | RAM: Kingmax 2x2GB DDR2 800 @ 833MHz| MoBo: MSI K9A2 CF v1.0 (BIOS: 1.D)| GPU: Asus HD 6850 1024MB (DirectCu) @ 835/1135MHz | Display: L24FHD | PSU: PC Power & Cooling Silencer 750 Quad | OS: MS Windows 3.11 Pro x64