Check the current system prior to upgrade to check compatibility, Microsoft has a Windows 7 upgrade advisor tool to run on the system prior to upgrade.
You will be best to remove antivirus and any other application suggested by the Upgrade Advisory tool (remember to reinstall latter)
If you wish to preserve all user settings then use the Windows Easy Transfer to copy all your settings to a removable storage device. (Windows Vista) (Windows XP)
With XP you will need to do a clean install of Windows 7 (The disk will be wiped), you can then use Windows easy transfer to re load your user data and settings.
Upgrading from Vista, if all goes well then all your settings and data should be already in place.
You will now need to get on line, if your wifi card is not working try the Ethernet cable, you may have had some warnings from the advisory tool about device drivers, the initial connection to windows update may fix some of these driver issues (I couldn’t believe it the first time I installed on an aging laptop)
I have seen reports of the upgrade hanging at 62%, guess what Microsoft have a fix! (you will need to edit a system Variable) KB975253
There are many guides and loads of advice on windows 7 on the web.