Hey has anyone ever experienced a problem with Oracle where the database instance will dismount after a period of time when there isn't any activity???

I have never experienced this type of issue before. If there isn't any activity in the database it appears to dismount and disconnect the user.

I can easily perform a startup mount and then open the database using SYS to resume whatever I was doing but I'd like to know how to prevent this from happening. Is there some setting that I may be overlooking that's causing this problem?

Points to consider: I am running ORACLE 10G SP2 on Windows Vista Ultimate with all Service Packs installed.

Any help or advice is extremely appreciated

thx