You say basic functions of windows?
Are you talking about Windows as an operating system (windows XP / Vista / 7 Etc) or talking about windows products?
You mentioned saving documents etc, I take it you mean Microsoft word etc? Thats a program, not a piece of the operating system (so to speak)
The operating system itself is basically the way the computer communicates between the application software / hardware, and the Display (The GUI - Graphical user interface), the folders, control panels etc etc.
Is there any functions you would like to do for example to give us an idea what you mean, or care to elaborate a bit more??
As for the virus, run this: http://www.malwarebytes.org/
Its free and pretty good!
Also free and worth a try is this:
Get back to us if you need more!