Here is my problem.For some reason whenever I try to net send a message at command PROMPT it gives me the message that "net" is not recognized as an external command or sumthing.
Ive used net send before and it worked fine but now for som reason its ACTING up.Whats wrong?What OS are you using? Commands seem to come and go on various flavors of Windows. Some XP commands do not exist on earlier Windows versions, Some Win98 commands no longer exist in XP, and MANY commands have new switches and syntax increasing functionality. If you are using xp, the MESSENGER service has to be running for net send to work.
If you want to turn messenger on permanently go to start>run and type services.msc scroll down to messenger right click to open properties and change start up type to automaticmake sure you also click start which becomes enabled AFTERWARDS... if i'm not mistaken, you do need to have start clicked before it decides to work for ya NET.EXE is a standalone DOS executable for Windows.
It may have been easily deleted.

Just look for the file from a similair version OS.sorry guys i wasnt clear ALL of the commands dont work now it cant recognise netsat,net user,net send or anything else! why is it doing this?Reinstalling the network adapter under Windows 98 will trigger a reinstall of the network tools like NETSTAT etc.

Under XP, you could opt to removing the network adapter. But remove the "Client for Microsoft Networks" first. Then remove the adapter under Device Manager.

Reboot PC. XP will re-detect the NIC and reinstall the Microsoft Netswork support itself. If no, you can do this.

This too will re-install the DOS tools for networks..

PS, not adviced for PC's that are members of a domain.