Open the Windows control panel and use the Mail applet. The first dialog
has three button, pick show profiles. From there you can add a new mail
profile. When you get the new one defined, you delete the old one.
This might help if stuck...
http://www.outlook-tips.net/howto/profile.htm
http://www.outlook-tips.net/beginner/editprofile.htm
In regards to the .PST file, it might be okay that it is larger that 2GB as
Outlook 2003 and later do have a PST file format that allows it to be up to
20GB. If the PST file is a carry over from earlier versions (Outlook
97-2002), then it is over the size limit of 2GB and is corrupt. So go the
route of creating a new mail profile with a new PST file. This will allow
you to continue sending/receiving new mail and then you can deal with the
2.7 gig pst file at a later time.