Firstlt you could try Firefox click here
No you cannot run IE6 on Vista to my knowledge and cannot remove IE7.
Windows Mail in Vista has had some issues,i have them too.
Best thing at this stage is to sit tight and wait for Vista sp1 update.
However,try giving the system a bit of a cleanup.
ccleaner click here
Winaso click here
Winaso has a trial basis of scan and repair any ten errors at any one time,though you can rescan and repair as you like.
Ccleaner,just use the default settings.
Any further help needed with these apps,just let us know.