If you have a Recovery CD from your computer manufacturer, the Recovery CD will install the Windows installation files (the Windows Cabinet file) to a folder, normally to C:\Windows\Options\Cabs or to C:\Windows\Options\Install. Open the Windows Explorer and look for them. Make sure you have the file Precopy1.cab.
Then, check that you have a cab file with a number (?????2.cab) all the way up to the last number. 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. A list of needed files are listed below.
Time was when you bought a new computer recovery disks for the OS were included. Nowadays its bad. Why? When recovery CDs were stopped and reserved partitions on the hd were initiated Rootkit malware started becoming more popular and the nastiest and most adamant of infections to remove, some (the best) reside in RAM and reinstall after you've nuked the hard drive. This is bad because the recovery partition is easy money to potential identity thieves seeking new victims. Why in the hell you put vital recovery info on the hard drive is beyond me - you increase your vulnerability or attack surface area... maybe not, because it makes money. You can use recovery CD/DVD from the vendor if your copies fail (buy them) or buy the more expensive microsoft OS but you still haven't covered your assets from evolutionary improvements in rootkits. Either way you end up with an always open backdoor to your computer where rootkits can reside, reporting your actions with keyloggers, installing trojans, etc and possibly stealing vital personal info which can at least give you a migraine and at worst ruin your finances and bring you much grief. My advice is to avoid as using your vital financial info online, use aliases, strong passwords, stay off unsafe sites including porn, use paypal or financial instrument separate from your main accounts, use vmware or something similar to surf the net, encrypt your personal info if you must keep it on the pc you surf the net with, maybe invest in lifelok, never store financial info on the pc that comes into contact with the internet - store it on other media you can physically secure, use a more secure browser like firefox with noscript and adblocker, etc, use linux, buy a REAL firewall in addition to antivirus. Now thats a lot aint it. Not really. Get smart about ID theft or risk becoming a victim. Educate yourself about rootkits and safe net practice.