Generally, OEM software can't be used for an in-place upgrade at all. It only allows installation on an empty partition.
Those who purchased Vista OEM software or computers over the past several months were offered coupons for special upgrade versions of Windows 7. Apparently these special OEM upgrades have the ability to perform an in-place upgrade of OEM Vista of the same version (x64, HP).
If you are not elegible for one of those free upgrades, you can always purchase a Windows 7 Retail Box Upgrade package. Those can upgrade your OEM Vista. Or you can buy a copy of OEM Win7 HP. But that won't allow an in-place upgrade.