In the old days it was undestood clearly that once you bought a piece of software, you could legally install it into as many computers as you wished, so long as you did not run more than one installation at the same time.
That is no longer possible, since the operating system checks the hardware configuration to cerify if it is a "legal" copy.
My desktop crashed. The fix was a new motherboard. That required a new hard drive since new motherboards no longer have two IDE connectors. Also new memory was required... bottom line, the ghost image from my previous desktop was rejected.
I keep three computers "current" all the time now at home. My "main" Desktop; a backup in case the desktop crashes (and it always does a few days before bills are due) and a laptop.
I never have the need to run two at a time other than transferring critical files for backup.
Will I pay for three full legal copies of Windows XP Pro? Nope.
It is possible to disagree with a law and obey it at the same time. You just choose not to.
But seriously, I do simmilar with my 98. I install on 3 hard drives then when it starts going buggy I start saving data to cd. When it gets too bad, usually after 2 or 3 months, I just pop a clean installed hard drive in.