That's a lot of questions, and at least you didn't make lots of topics, so thanks ;)
As for the administrator account, you could make it a limited user account, remove any passwords that are on the account and log onto it. The reason we make it a limited user account, is that if there is a virus that is set to start up, the virus creator probably intended for you to take a peak, and with a limited user account editing important files becomes harder.
Microsoft SQL Server is used by some programs for certain features, and if you understand some tech language you can find an explanation at http://en.wikipedia.org/wiki/Microsoft_SQL_Server
Visual basic is also required by some programs and games, for instance Flight Simulator X requires it before it can install.
Generally you do not need it, and if anything you will install in future does need it then it usually installs automatically with the program/game. However if not it is easily reinstalled manually. But try to avoid uninstalling them.
Many experts would recommend you have at least 15 GB free on your harddrive to ensure optimal speed, but it is much more effective and required for Vista.
If you believe you have a virus or infection then you can redirect yourself to the Am I infected? What do I do?
Or the HijackThis Logs and Virus/Trojan/Spyware/Malware Removal
Forum, with the help of our trained HJT Team.
Temporarily i would suggest you give MalwareBytes Anti-Malware a run through, as well as the ESET Online Scanner (Using IE only); which can be found at http://www.malwarebytes.org/mbam.php
(For the ESET scan ensure your using IE and tick the options "Remove found threats" and "Remove unwanted applications").
I hope i have been of some use to yourself, and will continue to help you if needed. Feel free to PM me