Looks like your computer is affected by

Virus Profile: BackDoor-CKB.gen.be

What you need to do is following:

1.Open up your pc

2.take out your harddrive

3.Spray some Raid on your Harddrive,clean it thoroughly using some bleach, and let it dry in sun for arnd 20-30 mins.You can also use some fabric softner for better outcome.

4. Place your harddrive back and turn ur pc on, see if its working, if not repeat from step 1.

This is how its done somewhat, even a kid knows..

Anyway jokes apart,

Looks like your explorer.exe(the application responsible for the taskbar, desktop, and Start Menu) is messed up.

Open up the taskmanager(which you said, you cant, i'll get back to it later)..

Goto processes tab, highlight explorer.exe, and click end process

Click File menu of taskmanager, click on new task(Run)

in the create new task dialog box, enter explorer and press ok..that should solve the problemo...

Regarding taskmanager, you might have some kind of spyware in ur PC, download free antivirus i.e AVG, and do a full computer scan with it, that should solve the taskmanager thing and also check if it solved ur other problem.

good luck..

post more if you have problems...

PS: I googled for u...i dont know much abt computers.

src:  http://lmgtfy.com/?q=windows+vista+startmenu+and+taskbar+disappear