when i log in to my computer, there is a red circle with a white x through it that says:

Your computer is is infected!
Windows has detected spyware infection!

It is recommended to use special antispyware tools to pervent data loss. Windows will now donwload and install the most up-to-date antispyware for you.

Click here to protect your computer from spyware!

the sign comes up when i ever i put the mouse over the red circle with the white x. it apparently downloaded something called PC Security 2009 to my Program Files. i tried deleting the folder, but it says that the program is in use and cant be deleted.

it happened 2 days ago. then stopped. then it came back again today.

what do i do?
thanks