Each version of Windows is different. Windows 95 and 98 have a file named Precopy2.cab while Millennium has a file named Base2.cab. Windows 98 has Base4.cab while Windows 95 has Win_04.cab and Millennium has a file named Net4.cab. So you need to look for the sequence of numbers, not the name. In all versions, all the Win cabs use an underscore before the number. The list of needed files can be found here.

Windows 95 normally goes up to cab file number 22 but your computer manufacturer may have added files, so you could have files up to 29 or even higher. Windows 98 has files up to 69. Here again, it could be higher. Millennium has files up to 22, again, there could be more. Each manufacturer has a different number of cabinet files. Once you have checked that these files exist, check that a setup.exe exists. Also, check for the file xmsmmgr.exe. If both of these files exist, then you probably have all the needed files to install Windows.
If not, you may have deleted one or two, or even had a FAT error and the files disappeared on their own. In which case, you may need to reinstall Windows just to get these files.