My task manager and regitry editor of windows xp has been disabled by virus. I have installed many softwares to re-enable the registery editor and task manager, and when I try to open it closes again.

Please tell me how to permanently re-enable both of them and to remove the virus that is casing the problem.

If you approach the problem from the other direction, the registry editor and task manager may not need re-enabling. First remove the virus and then see if further work is needed. Virus removal is somewhat of an art that cannot be distilled into a few simple steps. I strongly encourage you to take the machine to a local computer service center, but here is a general overview of my usual approach:

1. Boot machine into Safe Mode with Networking.
2. Scan machine with installed virus software. If software is needed, Malwarebytes Anti-Malware and Trend-Micro's online Housecall scanner are both free and combined recognize most malware.
3. Once malware has been identified by scanning, use the scanners to automatically remove what they will but note the malware names.
4. Reboot the machine into Safe Mode again and re-scan. While waiting for the scans to finish, look up the technical details and removal instructions for the specific malware at a threat database website, such as Symantec's. Also check for available removal tools for the specific viruses and download and run those.
5. Manually edit registry to remove any malware references in HKLM\Software\Microsoft\Windows\CurrentVersion\Run and any registry keys listed in the technical details about the viruses.
6. Reboot machine normally and login into each user account and repeat steps 2-5. Substitute HKCU for HKLM in step 5.

If a large number of infected items are found, consider it may be less time consuming and most thorough to wipe the hard drive with an erasure utility and reinstall the operating system and all software. Since the machine was infectable, either it lacked anti-malware software, up to date signatures, and/or the security updates (Windows Update) to thwart infection. Even if the malware is removed, unless additional steps are taken to secure the machine, reinfection is likely.