Below is copied from
http://help.lockergnome.com/vista/Import-Outlo ok-contacts-windows-vista-ftopict41096.html
Open Outlook and go to File, Import and Export
Export to a file
select Comma Separated Values (DOS)
select Calendar folder
select name and location of file and then Next
now go to
http://www.google.com/calendar/ and sign in or register
click on Settings
click on Import Calendar
select the file by browsing to where you just exported it
click Import
Now, Export the calendar by going to Settings again and clicking on Calendars
click on your Calendar name
in Private Address section, click the iCal button
this opens a window with a link to your ics file
click the link to download the file
Open Windows Calendar
go to File, Import and browse to your newly created ics file, make sure to update your existing calendar instead of creating a new one
click Import