Wiping free space

While wiping free space..which takes a long time..can I still use the computer on the net..while it's wiping

I would advise against it, although there's nothing physically to stop you. The way WFS works is to fill the disk with zero-filled files, then delete them. If you use the pc when the disk filling is taking place the response will gradually slow down and stop as the pc struggles to cope. So no, really.