Until recently, computers were supplied with recovery disks (CDs, DVDS) to restore the PC back to the state it was in when it left the factory, but this is now often not the case.
Now you are typically expected to burn your own recovery disks from software supplied on the computer, but by the time you are reading this, that is probably no longer an option. If you have or are getting a new PC or laptop ensure that you buy the Windows media or if not available make sure you make your recovery discs as soon as you get the new computer.
Increasingly, computer manufacturers now provide a recovery partition (a hidden portion of your hard disk where a copy of the original system configuration is stored), which largely performs the same function as recovery disks did, as such you should not try and "free up some extra disc space" by deleting what looks to be wasted space on your hard drive.
Trying to recover your OS and applications without your recovery disk(s) can cause a lot of problems, depending on the partitioning of your hard disk(s) and your ability to find appropriate drivers for all your devices. If you do not have recovery disks available, the following links may help you, depending on the make of your machine.
NOTE: Many recovery disks will completely wipe your hard disk.
Back up all your data before embarking on a system restore.
HP supply a recovery partition which can be accessed by repeatedly pressing F11 at startup. HP also supply the HP PC Recovery Media Creator with their systems which allows the creation of a set of recovery CDs. They will not supply recovery disks outside the USA or Canada.
Lenovo Thinkpads have a small orange key (or a key the same colour as the Thinkpad chassis) next to the power button, called the OneKey. Pressing this key will boot the portable into Recovery Mode. There is also a utility which can be used to make a Windows 8 recovery USB drive (but this does not not work with CD/DVD media):
Note: Some Dell XPS tablets have a particular problem with the system upgrade from Windows 8 to Windows 8.1. If you cannot safely restart your tablet and need to revert to Windows 8 for stability Dell advises the following:
"You may try to crash your system 3 to 4 times to access Advanced Recovery Options. To crash a Windows 8 system, you may power off the system while it is trying to load Windows OS (i.e. at Dell logo when the circling dots appear) by pressing and holding the power button down till the computer powers off."
Dell Recovery Disks (referred to as "Backup Disks")
How To Request Backup CDs for Your Dell Computer explains this option.
Toshiba no longer supply recovery disks with new laptops. They do provide a hidden recovery partition.
You can order Toshiba Recovery disks from https://backupmedia.toshiba.eu/landing.aspx
Toshiba also have a Toshiba Product Recovery Forum.
Samsung install a restore utility called the Samsung Recovery Solution which can be accessed by pressing F4 on startup. It will perform a complete or partial restore as described How To Restore Your Computer Using Samsung Recovery Solution. Restore disks are provided with the system.
Sony do not provide recovery disks. Sony recovery disks can be requested from Sony Technical Support, but they are reputedly not easy to get in touch with.
Sony VAIOs use the Vaio Recovery Wizard to access a hidden recovery partition. This can also be accessed by pressing F10 on booting. If the Vaio is so badly corrupted it doesn't boot properly the following may be helpful:
Acer offers a recovery partition (accessed by pressing Alt + F10 at Startup). It uses Acer eRecovery Management to enable you to create your own system images or restore from the hard disk.
Asus PCs have a recovery partition which on some models can also be used to create a recovery disk. The partition is normally accessed by pressing F9 at startup, and then Windows Setup (EMS enabled).
Various commercial sites offer to supply restore disks for various systems, but these should be the last port of call after the OEM manufacturer.