If you have created a new database recently, or have changed the init to increase the SGA you might run into a situation where the server does not have enough physical memory and virtual memory to open the database. Go to task manager and look at how much free memory you have and then look at the requirements of your SGA and see and much you need. You can always shrink the SGA a little to get the database started, or you can look for services that you can stop that you don't need, like Apache if you aren't using it, or you can just add memory.

If you are still having problem post your total physical ram, virtual ram and amount of memory used and your init file for your database.