Virus/Malware Help

I am not 100% sure if I have a virus or some kind of crappy Malware or something on my laptop. I let a friend of mine borrow my laptop for a couple weeks. I get it back and its not the same. I have a feeling he did something stupid and wont tell me about it.

I would just like to know of someone can guide me to the right place, where I can look up the basic/major areas of the registry to look for unusual things. Like if you look in a certain registry folder there should only be certain things in that folder. And if there is anything else in there then it may/may not be harmful.
Maybe the major reistry things to look for with the common malware/virus's that are around right now.
Like in the Shell Folder or the Run Folders etc..
I have looked in my programs and see nothing unusual in there to uninstall. But I know that there are things that are bad going on now I just dont know exactly what to look for specifically.
I am running Vista Home Premium. ITs an HP PAvilion laptop.

Any help is appreciated...

Oh yah.. I did look in a registry folder called
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders] and there was something in there that is called :
!Do Not Use This Registry Key

Is that supposed to be there? I just dont recall ever seeing that before...