If a program you have used has altered parts of the registry, check to see if it has a back up feature. I personally do not recommend registry cleaners as they are often more trouble than they are worth.
Before you enter the registry, you should always make a backup.
If you cannot do any of the above, you could try using system restore to restore your computer to a previous point before these problems have started.Restoring Windows XP to a previous State
To restore Windows XP to a previous restore point you need to open the System Restore Utility. To open the utility, go to your System Tools group under Accessories in your Programs menu. Then click on the System Restore icon.
You should select the radio button that is labeled "Restore my computer to an earlier time", which is designated by the red box. When that is selected press the Next button. You will then be presented with a screen similar to the picture below:
At this point you should select a restore point that you would like to restore. If a particular day has any restore points created on it the date will be in bold. You can then select the restore point by clicking once on its name, as designated by the red box in Figure 6 and then pressing the Next button.
At this point you will prompted with a confirmation as to whether or not you want to continue. If you do want to continue, you should press the Next button again, otherwise press cancel. System restore will then shut down all open applications and reboot the computer.
After the computer is rebooted you will see a screen that contains information as shown below confirming that the restoration to the restore point is complete.
If there are any problems with your computer since you restored to this restore point, you can revert back to your previous settings by going back into the System Restore Utility and selecting the "Undo my last restoration" radio button and pressing the Next button as shown below:
There are some problems associated with System Restore when it comes to viruses. When restore points are created they are stored in a directory that is accessible only to the System account and not to a user. This keeps the restore points safe from misuse and tampering. Unfortunately this also means that any virus scan software you may have installed can not scan the files located there as well. This causes a problem if a file that is infected with a virus gets backed up into a restore point because now the anti-virus software can not clean it. Now if you ever restore from a restore point, that file that is infected will be introduced back into your system.
With this in mind, if you find that you are infected with a virus, hijacker, or spyware and want to make sure you do not get reinfected if you restore a restore point, you should turn System Restore off and then back on again to clear all the restore points. This will guarantee that their are no infected files that could be restored.