What is the best software for testing customizing o.s.? vmware?, virtual box? , or virtual p.c. 2007?

I'm guessing from the fact you're posting in the "Other Operating Systems" that it is not a Win OS you are planning on running? Whilst MS Virtual PC 2007 should work ok, I think it would be safe to assume that its optimised for Win OSs.

Of the remaining 2 you mentioned, VirtualBox would be my preference for desktop testing/development. Having said that VMware server is available free and works quite well if you wish to have a number of VMs running. It really comes into its own if you wish test your VMs on an alternate PC (to the one your on) as it has a web interface rather than a normal window. Because of this, VMware Server lends itself nicely to remote admin.

Your question was possibly close to the order, except I prefer Virtualbox as it runs on practically any OS without too much trouble. VMWare seemed nice last I used it but can be at a cost. Virtual PC is a spot the version of Windows that It may work on i.e. artificial limitations added. Virtualbox is least restrictive ATM so has my recommendation.