Thanks, already managed to fix it with Time Machine Back-Up, however, you need only to restore the Safari.app file (can be found in Applications in Time Machine at a date before the last software upgrade to Safari 5.1)
So, practically,
go to Enter Time Machine
-> browse (using the arrows at right hand bottom) to a date when everthing was still working
-> go to Finder
-> go to the hard drive (probably named Macintosh HD)
-> go to Applications
-> go to Safari.app
-> highlight Safari.app by clicking on it
-> hold 'ctrl' key and click again
-> select from the drop menu that appears 'Get Info'
-> next verify that is written 'version 5.0.5'
-> close Info drop menu
-> make sure the file Safari.app is still highlighted and click on 'Restore' under the arrows at righthand bottom of screen in Time Machine.
Now you can leave Time Machine by clicking on Cancel on lefthand bottom
Safari 5.0.5 will be added to the Applications under Safari.app (Safari 5.1 will also still be there and has the word (original) written behind it) drag this (original) marked file to the trash.
That's all, works very good and means the whole system does not have to be re-installed.
Hopefully Apple will cure this 5.1 version quickly - Make sure always to make a Time Machine Back-up before updating software!
Hope this works for you all!