Check Task Manager:

Ctrl+Shift+Esc > Performance > Memory. How much is "Hardware Reserved"? Large amounts indicate configuration issues.

Check msconfig:

Type msconfig in Run. Boot > Advanced > Ensure "Maximum memory" is unchecked. This setting can artificially limit RAM.

Verify Windows Architecture:

32-bit Windows only supports 4GB maximum. Confirm you're running 64-bit for full RAM access.

BIOS Settings:

Check all RAM sticks appear in BIOS. Enable Memory Remap Feature if available. Consult motherboard manual.

Reseat RAM Modules:

Turn off and unplug computer. Remove and reinstall RAM firmly. Listen for the click indicating proper seating.

Test Individual Sticks:

Boot with one RAM stick at a time. Test each individually. Identifies if specific module is faulty.

Conclusion:

msconfig and BIOS are common causes. Improper seating or faulty modules require physical attention.