If you have a retail copy of Windows 7 Professional, as opposed to an upgrade disc, then you're best option is to do a clean install. By this, I mean reformatting the hard drive and loading the operating system on a blank slate. This will be more time consuming, but may be your only option if you don't have the upgrade DVD. I highly recommend you try using the Windows Anytime Upgrade first, though.
Before you reformat the hard drive and start installing the new OS, do yourself a favor and pre-download some of your drivers. Especially make sure to get any networking drivers like for Ethernet or Wireless connections so that you can get the machine back online after the initial install. The reason for this is that Windows 7 doesn't always include every driver needed to make every computer system work, and you may have to install certain hardware drivers separately. The problem is that if you can't at least get the PC online, then you won't be able to download anything unless you have a second computer available.
Visit the manufacturer's website for your PC, such as HP.com or Dell.com, and look for their Support section. You should be able to search for your specific model machine, which is often identified by a label or sticker on the PC, and then get the necessary drivers for your system. If multiple options are present, get everything and copy it to a USB flash drive or burn it to a CD or DVD. Better safe than sorry.
After you get Windows 7 Professional installed and the system is online, be sure to check Windows Update and get everything that is available before you start loading other software. These updates will make your system more stable and reliable, plus its best to get them out of the way to make sure everything is working right before you start personalizing Windows.