Based in part from this comprehensive article on The Elder Geek.

XP repair/backup tools (some also appear in earlier versions of Windows):

1. Backup

Utility supplied with both Windows XP Home and Professional (only installed by default in Pro). This utility allows you to select the folders to be backed up, choose a destination and set a regular automatic backup schedule.

When using this it pays to understand the different kinds of backup available. A common approach is to schedule normal backups with differential backups in between. To restore such a backup you only need restore the last normal backup and then the last differential backup. If you use incremental backups restoration can be much more complicated.

Back up types in a nutshell...

Every file has a flag or attribute which tells the system that file is ready to be backed up (or archived). This is the 'A' attribute. Any time a file is changed or renamed then the 'A' attribute is set to 'on'.

A normal backup will back up all files regardless of their 'A' attribute state. It will also set this attribute as 'off' for all files that it backs up.

An incremental back up will only backup files with the 'A' attribute set as 'on'. It will reset this attribute to 'off' on these files. If you have a regime of normal back ups with several incremental back ups in-between then you'll need to restore the last normal back up followed by every incremental back up since to do a full restore.

A differential backup will back up all files with the 'A' attribute set as 'on', but unlike the incremental back up it will not reset the attribute to 'off' for these files. If you have a regime of normal back ups with several differential back ups in-between then you'll need to restore the last normal back up followed by just the latest incremental back up to do a full restore.

More on back up types here.

2. Device Driver Roll Back

Removes a newly installed device driver for the selected device, returning to the previous driver. Doesn't work for printer drivers. To use this, right-click the device in Device Manager > properties > driver > rollback.

3. Disable A Device

Use this to take out a device altogether, to help in diagnosing problems. Right-click the device in Device Manager > Disable.

4. Add/Remove Programs

5. System Restore

In theory this "Restores all system and application settings back to those that were in effect at a point in time you specify. Restores key application, driver, and operating system files changed since that time." And it leaves all your data files intact!!
Sounds just dandy, eh? Yes.....if you could trust it.
DO NOT RELY ON SYSTEM RESTORE!
As it says here, "System Restore points get deleted for many reasons, making it unreliable, especially in the long run"
System Restore can be found in Start > Programs > Accessories > System Tools

6. Last Known Good Configuration

Restores registry and drivers that were saved last shut down, so is only any use if you realise you cocked up big-style in the current session. Here's how to use this tool.

7. Safe Mode

If this didn't exist the suicide rate among IT guys would soar!

8. Recovery Console

You can run the Recovery Console from your Windows XP cd or install it on your system for later use. To launch it from CD, start the PC with the CD in and hit 'R' when prompted.

Recovery Console isn't beginner friendly, using as it does a command-line interface. Here's Microsoft's list of commands you can use, and here's an example of using the console to extract a system file from the CD.

Good luck!

9. Repair Current Installation

Boot from your XP CD (tweak your BIOS to boot from CD first if neccessary).
Follow your nose until you are offered 3 options. Pick option 1 (not the Recovery Console option).

After you do this you must remember to re-download all neccessary Windows updates.

10. Reformat hard drive, reinstall windows, all software, drivers etc.

The nuclear option. Sometimes it's the only option!