Ubuntu is a great "starter" linux, and truth fully you dont need to move onto anything else
what i would suggest... (my setup)
1install windows configure with appropriate drivers and antivirus (recommend AVG)
2 install ubuntu (very very easy, literally just a few clicks)
this allows a dual-boot mode to windows (defualt being linux) so you can run those windows apps
HOWEVER for the fullfledged linux user you can
1 use a virtual machine to emulate windows (great alternative)
2 my personal favorite Wine, its not a windows emulator, it simply allow execution of windows program over linux kernel (with no loss in speed at all)
now to your questions...
1. NO you cant run ALL but you can run ALOT of them (you can run ALL with a virtual machine)
2. yes, but there are very few viruses for linux to start and ubuntu is security updated constantly so if there IS one you most likely wont get it, and yes its easy to fix, even if you have to delete everything but your files and reinstall all the originals (there are guides for this) but thats really a last resort
3. its free, and OHH YES its worth it, esspcially if you get it set up good, its faster\smoother\Faster\nicer-looking\ and FASTER than windows by far
4. hard to say... i would say
Pro:
Free (and legal
)
install applications without downloading them from a site (running a command like this 'sudo apt-get install "application-name"') most applications have a list preconfigured in ubuntu hardy
Cons:
truthfully the only thing i would say IS its not windows, as in it wont have the same support, but to me thats not really a con
what i suggest is download the iso and burn it to a disc
boot to the livecd (its very simple) and test it out without installing it yet, full functionality including the ability to install apps!