HELP! My screen is stuck on 800x600 and won't let me change it!

Hi Everyone: I'm totally new to this and signed up because you all sound like geniuses after reading some of your postings. Anyways.

My screen is stuck on 800x600 setting and when I go to change it, the setting reverts back. I can't go into the Advanced settings either. Based on some of the postings i've read it sounds like something to do with a video card? Except I don't know what that is, or how to install/reinstall or whatever it is I need to do. Please HELP me as I'm going crazy staring at this screen! Thanks in advance for any insight...

as requested for new members here is more info on my computer-

Computer:
Operating System Microsoft Windows XP Professional
OS Service Pack Service Pack 2
DirectX 4.09.00.0904 (DirectX 9.0c)
Computer Name SUPERVIXEN
User Name Inga

Motherboard:
CPU Type Intel Pentium 4, 1700 MHz (17 x 100)
Motherboard Name Asus P4T-E (5 PCI, 1 AGP Pro, 1 CNR, 4 RIMM)
Motherboard Chipset Intel Tehama i850
System Memory 1024 MB (RDRAM)
BIOS Type Award Medallion (12/12/01)
Communication Port Communications Port (COM1)
Communication Port Communications Port (COM2)
Communication Port Multiport Communications Port (COM3)
Communication Port ECP Printer Port (LPT1)

Display:
Video Adapter Matrox G550 (Matrox Millennium G550)
3D Accelerator Matrox G550

Multimedia:
Audio Adapter Creative SB Live! Player 1024 Sound Card

Storage:
IDE Controller Intel(R) 82801BA Ultra ATA Storage Controller - 244B
IDE Controller M-Systems DiskOnChip 2000
Floppy Drive Floppy disk drive
Disk Drive Apple iPod USB Device
Disk Drive ST340016A (40 GB, 7200 RPM, Ultra-ATA/100)
Optical Drive CD-ROM Drive
Optical Drive CD-ROM Drive
SMART Hard Disks Status OK

Partitions:
C: (NTFS) 38162 MB (14904 MB free)

Input:
Keyboard Standard 101/102-Key or Microsoft Natural PS/2 Keyboard
Mouse PS/2 Compatible Mouse

Network:
Network Adapter D-Link DFE-530TX PCI Fast Ethernet Adapter (rev.A) (68.148.207.82)

Peripherals:
Printer Microsoft Office Document Image Writer
USB1 Controller Intel 82801BA(M) ICH2 - USB Controller 1 [B-4]
USB1 Controller Intel 82801BA(M) ICH2 - USB Controller 2 [B-4]
USB Device USB Mass Storage Device

Sounds like you just need to install your video drivers. You can get those here