Windows XP, Vista or 7 cannot natively open a .iso in Windows explorer like Windows 8.1 can. VirtualClone Drive is a free utility which gives this feature.
Virtual CloneDrive can be downloaded from here:
Launch the .exe and accept the UAC:
Next accept the license agreement
Select "Always trust software from Elaborate Bytes AG" and select install
Once the install is done, select close.
Now you should be able to open .iso file in Windows Explorer by right clicking the .iso and selecting Mount (Windows 8.0 and 8.1 have this ability natively):
Select open folder to view files:
The .iso now can be opened in Windows explorer.
A program like ImgBurn can be used to copy all old game discs and software CD/DVDs to a .iso image which can be stored on an external hard drive. Virtual Clone Drive can be used to run them.