How do i know my computer is infected with virus

Generally when computer starts taking longer than usual to load some application you get a feeling that computer is infected by some unwanted code. But it is not at all necessary that these problems are due to virus as these may be some software or hardware problem so check them.

So now how to ascertain problem is due to virus. Few signs that may help are:-

* When you click on any of the local drives they show Open With window.
* You observe certain file changing its size in seconds from small to big and back.
* Unusual error messages.
* Operating system slows down.
* Disk space 'vanishes'.
* large number of unknown .exe files in principle drive (usually in C:).