Is check for spyware - It could be another program or your virus program so we will need to know what programs are being loaded at startup - you can find these in msconfig.
Then you need to tell us the runtime error number - you can also goto "event viewer" (admin tools) and look in the System and Programs folders for any "red" error messages - click on them and follow the link to the online "help and support" centre. Report back what you have seen if you are unable to fix the problem yourself.
Also let us know the history of the problem eg when it started, how long you been using Outlook, have you had or having any other problems or crashes.
You can try the following in the meantime
1/Find a file called SCANPST and run it - it will fix your outlook database - first backup the databases to another temp fold.
2/The next is to reset your mail account - You will need all your email settings for this method, so have them handy (you will find these in your existing account settings) - then:-
Open Control Panel > Mail > Show Profiles
Add a new profile and choose "Always use this profile"
(you can change this later if you want - Then Apply and Close.

Let us know how you go and supply info if you still having problems.

Peter