I have been given a company computer and I wish to clean the hard drive, how do I do this. I am not yet connected to an internet provider

How "clean" does it need to be?

See this post for links to complete drive erasure utilities. After using this type of utility, the operating system, drivers, and application software will need to be reinstalled and configured to have a usable computer. Lack of an Internet connection will complicate obtaining one of these utilities.

Another possible option that may exist since it was a business computer is the computer may have a recovery feature that restores it to the as-shipped state it had when it was new. The documentation for the computer that may be available on the manufacturer's web site should indicate how to use this feature if present. Normally, a certain key sequence is pressed at boot, perhaps in conjunction with recovery CDs, to invoke a wizard that steps through the recovery process.

Otherwise, to keep the operating system intact, the only option is to manually delete unwanted files and uninstall software that isn't desired.

If you want to totally wipe drive clean, dban will do this

Darik's Boot And Nuke | Hard Drive Disk Wipe

This will take hours depending on how big drive is

After this is done, you would install operating system from scratch

How to Format Drive and Install Windows from Scratch
Geek to Live: How to format your hard drive and install Windows XP from scratch

Installing Windows Vista
Installing Windows Vista - NeoSmart Technologies Wiki