Deal with the disk space first
The important thing is that you verify whether or not you are actually low on space and then deal with getting Vista more. Without enough disk space to keep your OS from choking, you will likely have issues installing the tools that you need to fix any spyware/viruses that you may have.
1) Open "My Computer" and see how much space there is.
2) If it is low, RIGHT click on the drive in question and then LEFT click on "properties."
3) From there, click on the "disk cleanup" button and let it run.
4) You will see a list of items that Windows can remove to clear space. I would choose all of them EXCEPT "compress old files."
5) If you still need more space, go to Control Panel--->Add/Remove Programs and see if there are applications that you can remove to free up more space. NOTE: If you don't know what a program is for, Google it first or ask! Some programs with boring names are actually important!