Is this in XP?
If so try this.....
1. Go to Control Panel/User Accounts.
2. Select 'change the way users log on or off'
3. Is 'use fast user switching' ticked? If not, that probably it.
If it's already ticked, how much ram do you have in your pc? If it's less than 512mb it may be that it hasn't got enough 'oomph' to run both accounts at the same time.