Inspiron 1750 troubles mounting

An escalation of trouble has me seeking the advice of you guys,here's the story................

I was thinking about upgrading my memory and HDD as my 1750 has no upgrades since new so I went to Crucial for a scan for compatible memory and an HDD which it gave me BUT the scan showed only one of my memory banks was in use. I know they both are and that its remained untouched since new. I checked they weren't loose etc and thought it was puzzling and that I'd research the problem. Later I tried to start up as normal but windows wouldn't load, I ran windows diagnostic which read "a critical file was missing or corrupt"........I tried safe network with command prompt and nothing would work,I cant do a factory restore. It wont run my CDs which I got with my laptop Can anyone assist, my files are backed-up regularly so there's nothing I can loose.