The months before a new operating system ships can be like the calm before a storm – things go quiet as people wait for the big event.

For PC makers, such pre-storm lulls can have a significant hit on sales. Faced with the choice between buying now or waiting for the gleaming new software, many customers opt to keep their cheque books firmly in their pockets.

But, in an effort to avoid such pre-launch slumps, Microsoft has previously offered free upgrades to those buying a new PC just before the new OS hits the streets. This move has been welcomed by consumers and small businesses that need a new device immediately.

But the generous offers have always come with a restriction on the number of PCs that can be purchased and later upgraded. While this is no big deal for those purchasing just a couple of machines, what happens for larger companies who need to buy many more?

Last week Microsoft announced that the upgrade option for those purchasing Vista-based machines and wanting to move to Windows 7 when it ships will be limited to 25 PCs or fewer.

Companies contemplating larger purchases need to do their homework. According to Gartner analyst Michael Silver, one option is to enrol the new machines in Microsoft’s Software Assurance program which covers future upgrades of the OS and applications. However this option will cost extra and needs to be added to the total purchase price.

A second option, says Silver, is for companies to lean on their technology provider to encourage them to throw in extra free Windows 7 upgrades as part of the hardware purchase deal. It seems larger channel partners have the option to do this, and if it means getting a big order over the line they might just play ball.

The third option is to simply delay the purchase of new PCs until after the October shipping date for Windows 7. If you're a big organisation and can wait that long, this might be the best and most cost-effective long-term option.