I certainly understand what it's like to have arthritis. Remember that Google is your friend, although I use SSL Scroogle which stops tracking by the big boys.
As to your need of an Invincibility Mod for GTAIV there's lots of different mods out there and the list is growing. The following address at YouTube doesn't make you immune to damage from falls and stuff but it's a hoot to play with weapon battles:
http://www.******************?v=fbNXYjKDK6E Check the (more info) under the author's name on the right.
Also, remember to scan the files you download with an antivirus before opening them on your PC. Just right click the downloaded file and choose from the options menu that pops-up. Once bitten, twice shy.
Btw, you can survive a fall even from great height in the game if you bounce off something else before you hit the ground. And if you're on the motorbike, release the accelerator before you land. You'll stay alive and won't wipe out.