don't do it. my story:
ok, so a laptop bought in the middle of last year came with a free upgrade to windows 7. excellent - the ability to future-proof help with the sale.
of course, being an hp laptop meant that it came with a fair amount of bloat-ware so it wasn't particularly quick to boot. it was livable, but experiences with win7 on another machine caused the urge to benefit from the new interface on the laptop.
after christmas it was time to do the upgrade. reading about in-place upgrades led me to believe that ms has got their act together with the vista to win7 in-place process so I decided to try it.
let's just say that with all the backing up and the upgrade itself, it took 2 days... yuk! still, at the end there was windows 7 installed on the machine! pretty, and functional. unfortunately, a previously slow boot had been significantly slowed, leading to a boot time of at least 10 minutes. and then it developed a regular blue screen associated with sleep/hibernate... from bad to worse!
nothing for it but to rebuild the bugger.
a clean win7 installation is less than 30 minutes - impressive. of course, recovering user data & getting all the apps installed, updated & configured is the real trick. add another day and a half and the new installation is usable. by not installing hp's bloat, the boot time is also far improved.
a properly working computer is a beautiful thing...