If your laptop gets really messed up and you would like to start from scratch, each manufacturer has a different procedure for reverting to the fresh-from-the-factory state or doing a clean install. The following list is not complete, but is as accurate as I can make it from my personal experience.
Dell -- Older Dells came with CDs for the OEM version of Windows, drivers, and some of the installed applications. Starting in mid-2004, Dell has been shipping their laptops with a utility called Dell PC Restore by Symantec. This utility is on a partition on the hard drive and can be triggered by pressing Ctrl+F11 at the Dell BIOS startup screen for XP laptops, and by pressing F8 after the BIOS screen (like you would to get into Safe Mode) and choosing the system recovery option on Vista machines.
Toshiba -- Older Toshiba laptops came with a restore DVD with the original factory configuration on it. Newer ones come with a recovery partition that can be triggered by pressing and holding the 0 (zero) key as soon as you press the power button, and the releasing it when you see the Toshiba logo.
HP/Compaq -- Older laptops from these manufacturers shipped both the original factory image on a four-CD set and an XP OEM reinstall CD. However, I have found the restore CDs to be buggy, and in my experience they often don't work (they often freeze at the same point each time). Newer ones use a recovery partition that can be triggered using F10 or F11, depending on the model.
IBM/Lenovo -- All of their recent laptops can be restored by pressing the blue Access IBM or ThinkVantage button at the top of the keyboard at the IBM BIOS boot screen. This will launch you into a multipurpose utility; you'll have to choose the restore option. This is a lengthy (2-3 hour) process; leave plenty of time for it. The recovery utility is on the hard drive; you can receive backup CDs for free if you request them within the first thirty days or if your hard drive dies while under the IBM warranty, or you can burn discs yourself using a utility.
AFAIK, here's no way to get a regular Windows CD from them.
Sony -- Sony is another manufacturer that puts a restore utility on a hard drive partition. You can access it by pressing F10. Sony's recovery CDs (if you accidentally erase the recovery partition) are very expensive (although you can burn a backup yourself), and doing a clean install is often not an option becuase they don't provide drivers for all their proprietary hardware.
Gateway/eMachines -- Newer Gateways (and I would assume eMachines too) include a recovery partition/utility that is triggered by pressing F11. You can make a backup of it. I have seen their driver support to be somewhat spotty, although not as bad as Sony's.
Acer -- They have a utility called eRecovery that you can use to restore the system. It can be triggered from within Windows or by pressing Alt+F10. They provide a Windows utility to burn CDs.
Averatec -- They provide restore CDs, but I'm not sure of their nature.
Micron -- I've never seen the restore process on these laptops, and I couldn't find much information on their site, so I don't know what it entails.
If anybody has any further information, feel free to add it.