Unless you deleted it, you should have a recovery partition. When the system is booting you should have an option to go to a boot menu, which should allow you to boot to the recovery partition.
If you buy a full version of Vista now, I don't know if you will qualify for the upgrade to Windows 7, or if that upgrade option is only for computers with Vista installed. Vista is the only Microsoft OS currently available unless you have a subscription to Technet or MSDN. So if you can't reinstall from the recovery partition, your choices are:
1) Purchase a full version of Vista.
2) Try to get recovery disks from the manufacturer and install Vista. You will be unable to use the computer until you get the disks.
3) Wait for Windows 7 to come out and buy a full version of that, but not be able to use the computer till then.
4) Install Linux either as a temporary measure until Windows 7 comes out or as a permanent replacement for Windows.