You know when you go to close down your PC and the desktop picture goes to Black and White, is there anything you can download or any site that you know of which enables you to do this to other pics on your PC?
If you mean that you have colour pictures stored on your PC that you wish to convert to greyscale, then most graphics packages have this as a standard function but I would suggest that Irfanview is well worth the download as it is both free and packed with useful features for viewing/editing pictures...
kempie's mentioned the exact command that's available in Irfanview. Under the "Image" menu when you've opened a file, just choose "convert to greyscale".