Hi
This does look like you have lost ownership on that specific registry key.
The first thing to do is to take a registry backup, just in case something does not go to plan in the detailed steps below. Please take a full backup of the registry, this is fairly straight forward to do.
Refer to this Microsoft web page.
http://support.microsoft.com/kb/308422/If you have XP Home then you may have to install the backup tool first, instructions are in the above web site.
Save any work and close all running applications. Then run the backup tool as directed in the above web site. You do not need to backup any folders but you do need to Click to select the
System State check box as desribed in step 3 in the Microsoft web page.
If you are unsure of any steps there is a handy video on the above Microsoft web page that shows you the steps, you just need the System State check box ticked.
Once the backup is finished start regedit:
click start select run and enter
regeditand click ok to start regedit. In the left-hand column click the + sign to expand
HKEY_CLASSES_ROOT then scroll down to
mailtoHKEY_CLASSES_ROOT is a large entry in the registry so it will take some scrolling to reach mailto
select the mailto item, and then on the edit menu at the top of regedit select permissions. Then click the Advanced button, then Owner tab.
Under the area "Change
owner to:" you will see a list of user names, click to select your account name and check the box "Replace owner on subcontainers and objects" then click OK.
You now have ownership of the key so can set the permissions correctly. Still with the
mailto key in the edit menu select permissions again.
Under the section called "group or user names:" select one entry at a time, you will need to do all in this list one at a time:
Creator Owner
Administrators
Users
SystemPlease check that all the above are actually listed in the list, there may be others but all these should exist.
Then select each in turn and for each one check the "Permissions for Users" area below have the following settings checked, if not check the appropriate boxes and then click OK
Creator Owner should have
Full Control
Read
Administrators should have
Full Control
Read
Users should have
Read
System should have
Full Control
Read
If any are missing check to enable the permission and click OK to save. When finished close regedit. Then reboot the PC and retry your mail program, does this cure the issue with multiple windows?