Solidworks, pre '10 release, had a floating license option using their network licensing. Essentially, you could use the software on any computer in the network it was installed on, but the same license or ‘seat’ couldn’t be used at the same time.
Cyberdemon is correct, you can write off the software expenses, and you should. Not 100% of it, but a nice chunk of it. As a freelance designer, you should learn about that.