Ms Office Home And Student 2007 Wont Install?

Dell Inspiron w/Win 7. MS Office worked fine this week. On 1/22/12 I contacted Dell re: blue screen problem. Tech said it was because my computer was running too slowly and he did cleanup, etc. Then he installed System Mechanic and told me it would solve problems. Had even more problems so contacted Dell last Sunday (1/29/12) and THAT tech told me System Mechanic should never have been installed because it conflicts with my anti-virus and told me to uninstall which I did 1/30/12 after conv. w/Dell about refunding my money.Discovered my AV had not updated at all during the week. Contacted Total Defense today...spent 4 hours w/him to uninstall/reinstall and get it working.NOW none of the MS office programs work. All say they're not installed, but they are there in the program files. Tried to reinstall from CD...quit half way through and said it could not finish installation. I used Excel during the week so I know it was working after System MEchanic install but do not know if the uninstall of that or the changes the AV tech made are responsible