Recently, my computer absoƖute refused to start-up. A friend got it started, but certain programs are missing from the "add/remove" list

1. Avira AntiVir Personal - Free Antivirus
The following error occurs when trying to start the update:-
"Scheduler not loaded".

2. Safari:-
When I try to open "Safari" I get:-
Problem with shortcut.
This action is only valid for products that are currently installed.

3. Malwarebytes.org

When I try to open it, I get:-

vbAccelerator SGrid ll Cont...

Run-time error '0'. On clicking OK I get:-
Run-time error '440'
Automation error

None of the above are in the "add/remove" list, but are still lurking somewhere as none of the same can be re-loaded.

4. iTunes:-
When I try to open it I get:-
"This action is only valid for products that are currently installed".
Also "apple application support was not found", try deleting iTunes and re-install. I could not delete iTunes from the add/remove list.


5. Skype
When I tried to open I get:-
"This action is only valid for products that are currently installed".
However, I was able to re-load Skype and it is functioning properly, although it too was not in the "add/remove" list.

6. Gmail/Yahoo
I am using Internet Explorer 8 and am able to send and receive emails, but cannot view "you tube" clips and do not get any error codes.

All of the above were working fine before the incident, these are the only ones I have tried.

I hope someone can help me.

By the way, this is my seventh or eighth attempt to send this. My previous attempts were balked each time I hit the submit/preview button, alleging that I was not logged-in. I could not understand why, as I had to log-in to get to the forum and post and am sure that I did not intentionally log-out.