I am used to reformatting my PC and feel that it is important to do so at least once a year, but I usually do it twice. I am always trying out new software: installing and uninstalling, and installing again... This in time really messes up the whole internal working of your registry and hard drive leaving a lot of stuff there that did not uninstall and also takes stuff out (sometimes) that is needed by other programs. It can be quite frustrating at times. Once it gets to bad I just reformat.
1) I use a program called Wipedrive 3.0. This program runs from CD at reboot. With it you can completely wipe your drive, even to DOD standards (3 times rewrite). The technical is not important, but what is important is that you completely rewrite the drive. This prevents "ghost images" remaining that can possibly interfere later after a normal "windows reformat" which really just tells the computer that the drive is empty when it is not. (sorry- non important technical info)
2) After you have done this you will need to repartition your hard drive. I use a program called "Partition Commander" which also works by CD from reboot. I usually partition my drive with at least two partitions. one for the operating system and another for all my stuff. This way if I have to reformat, all I do is back-up MS Office, my EI7 Favorites and the 'my stuff' partition. Much easier and can also save you if you get a virus that causes you not to be able to boot the C: drive. You can then hook the hard drive to an external USB and just access the 'my stuff' partition. (once again to much info, but stuff I feel passionately about) Also make sure that the new partitions are in NTFS format.
3) Reinstall operating system, using the existing partitions that you created with Partitioning software rather then letting Microsoft create it. Then reinstall all your software’s.
Regarding not having operating system disk. I had this problem way back and just went to the website and requested a new operating system disk. They will usually only charge shipping for this. You can also find a friend (or enemy) with the exact same computer as you and their disk should work just fine. You just need to use the Microsoft Key from the sticker attached to your computer. If it is a non Vista OS then (if your computer can handle it) I recommend upgrading to Vista Home Premium. I use Home Premium on my desk top and the Ultimate version on my laptop. Ultimate has a lot of extras that the average user does not need though. Home Premium is an excellent OS.
The best way to buy Vista is a OEM copy that you can find online for the same price as the equivalent Vista Upgrade. This way you can install a fresh version from scratch rather than upgrading. If you cannot find an OEM version for a good price then you can install the Vista Upgrade as a trial. Then from the desktop run the Vista Upgrade again, this time putting in your KEY, and it will reinstall it over the trial. This takes a little longer but works fine. It will save the trial install in a folder called 'windows.old'. Delete this once you have finished the upgrade.
I hope that this is not to long winded. It is a lot of info, but I think most would apply to your situation.
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