I use Vista, which is not so simple to fix as Xp was in IE.
In Vista you click on Default Programs from the Start Menu, then Set your default programs and see if Notes is listed, mine is not(but I run R3-R8 so not surprising it can be confused).
Then check Set Program Access and Computer Defaults to see if Notes is listed as an option, click it if it is.
In my case this still did not work..
The answer was/is:
Go to this page and get his tool.
You can follow the steps, but in Vista I had to go into the registry and look for in the
and change it to your notes.exe location and add the ending. mine is c:\lotus\notes8\notes.exe -defini %1
There are other entries ie: rss and mail inks and all which you may need to fix as well, but this will get the email working again.
EDITED UPDATED 3/20/2008
Jospeh thanks for this information for Thunderbird people too!
Also, my key for Lookout was
And the full entries from that lovely program you pointed me to were:
Microsoft Outlook.Url.Mailto Enabled URL:MailTo Protocol C:\PROGRA~1\MI1933~1\OFFICE11\OUTLOOK.EXE -c IPM.Note /m "%1" Microsoft Office Outlook Microsoft Corporation
Notes Enabled URL:Notes Protocol C:\Program Files\lotus\notes\notes.exe -defini %1 IBM Lotus Notes/Domino IBM Corp
Thunderbird.Url.mailto Enabled Thunderbird URL "C:\Program Files\Mozilla Thunderbird\thunderbird.exe" -osint -compose "%1" Thunderbird Mozilla Corporation