may be permission problem
Run regedt32, open the key HKEY_CLASSES_ROOT\ProgID, select Permissions from the Edit menu, and grant the "Everyone" account Full Control over the key
check this link
http://forums.asp.net/t/1127066.aspx
c:\> regsvr32 c:\path\SMTPMail.dll
2.On Windows 2003 and XP, this error may also mean the component's key in the system registry has been assigned insufficient permissions. Use regedt32 to give sufficient permission. Select Permissions from the Edit menu, and grant the "Everyone" account Full Control over the key.