Are these the symptoms of any known virus?

First of all, I'm not very good with computers so please bear with me.

About a month ago, my PC restarted for no apparent reason. I then noticed that so many things have changed dramatically. There's too many to list, so I'll just mention the ones that stick out the most:

+ Appearance changed: it went back to windows classic (from the original Vista appearance). I was able to change it back to Vista.
+ Statup programs have all disappeared and won't come back. I tried so many times to have start-up programs reappear, but it just won't work. Now when I click the "start" button, the part where programs used to appear (google chrome, MS Word, Firefox...etc) is completely blank.
+ When I try to make certain changes, then won't take effect. Ex: open programs of the same type get grouped together and I don't like that. I went and unchecked that feature, clicked apply, nothing happens. It still groups them together.
+ Default programs changed: windows media player (WMP) used to be my default media player. Now, for videos, Divx is my default media player, iTunes player is my default MP3 player, and any other type that used to be assigned to WMP is now an unrecognizable file. I try to pick WMP as my default player several times, using various options, the change just won't take effect.
+ Recycle Bin is ALWAYS empty: if I delete a file, it won't go to the waste basket first; it gets automatically and permanently deleted from my PC.

I'm sure there are more issues that I haven't discovered yet, but this is enough to really piss me off.

I've had PC problems in the past and I've always been able to resolve them myself. But this is the very first time that I've encountered something that I just don't understand or know how to deal with. I've ran two or three different virus/malware scans and NOTHING was detected.

I've now come to humbly request the assistance of those of you who may know what's wrong with my PC (perhaps even recognize the symptoms I mentioned) and suggest a solution.

I think I'm supposed to post my PC "log" or whatever it's called and I don't know how to get it. If I remember correctly, I'm supposed to download something called hijackthis and use it to generate a log, is that correct?

Anyway, your help is much appreciated.