To take over the control the remote XP computer, click the Take Control on the top right. If you can’t take the control, make sure Allow helpers to remotely control the computer under Offer Remote Assistance Setting is enabled. By default, Offer Remote Assistance Settings is disabled. To do enable it, open Group Policy by running gpedit.msc. go to Local Computer Policy\Computer Configuration\Administrative Templates\System\Remote Assistance. The user who will be giving assistance must be a member of the Local Administrators Group on the receiving machine or you need to added as a Helper in the Offer Remote Assistance Group Policy Setting. To add User and Groups to Group Policy: Go to Offer Remote Assistance Group Policy, and in the Helpers area, click Show. Click Add and then enter the Domain\user account

In a case you want to remote access a Windows XP professional workstation and the computer Remote Desktop is not enabled, or no one over there to help you to enable it, you may have an option to enable Remote Desktop remotely by using regedit.

To enabling Remote Desktop using regedit, follow these steps:

1. Run REGEDIT from Start>Run
2. Click on File, then select Connect Network Registry
3. Type the remote computer IP or host name in the Enter the object name to select and the click OK.

4. If you don’t have permission to access the remote computer, the logon screen will show up. Type the username and password for the remote computer. Then click OK.

5. Now, the remote computer is listed in the Registry Editor.

6. Browse to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Terminal Server, in the right panel, select fDenyTSConnection (REG_DWORD). Change the value data from 1 (Remote Desktop disabled) to 0 (Remote Desktop enabled).

7. Close the regedit.

Now you can use remote support