Hi Sandra. What you ask is likely to be difficult.
I don't know what your computer system is, but if it is an older XP system which you have upgraded to Windows 7 then you can simply reformat the disk and reinstall XP from the original XP setup disks you have. But, all current data will be lost.
If this is a new system with Windows 7, then downgrading will be beset with problems. Firstly the system may not be compatible for XP anyway. If it is, you can only reformat the disk and install XP, and again all data will be lost. But then you have to hunt down all the drivers that XP will need for the motherboard, sound card, video card, network cards, SATA drives, and so on. The list may be endless.
If this computer is supplied by a manufacturer, eg Dell HP, Packard Bell etc, their web sites may offer a 'downgrade path' for your model with drivers and guidance, but it is likely to be a tortuous process.
Which version of Windows 7 do you have? If it is Windows 7 Professional or Ultimate, they both have options for downloading and installing from Microsoft "XP Mode", which may allow you to install those games and play them. But this is not available with any lower version of Windows 7.
Can you tell us more about this problem. You mention Installshield Wizard, but not what happens.
Have you tried downloading the game and saving the file to your desktop, (do not download and run), then using Windows 7 Compatibility Mode? More about that here;
http://windows.microsoft.com/en-US/windows-vista/Make-older-programs-run-in-this-version-of-Windows
Mark