Currently I have a all tasks linked to one or the other project. Some users want to create personal tasks so that these are not linked to any other table. Without having to create a new table, is there a way I could use the existing task table for personal tasks?
If a requirement right now is for a task to be linked to a project, you could set up a "dummy" project and call it something like "Employee Personal Tasks" and have them link to that, perhaps with a special button that hard-codes its Record ID and automatically fills in that piece for them.
You would just stuff the value of the Related Project with the hard-coded Record ID# of that dummy project so that nobody needed to fill it in or even think about it.
If these types of tasks are much simpler than the normal project tasks in terms of what other fields are necessary and relevant, you could even set up a different form (clone the original) and then remove what's not necessary, and then part of your button formula would be to specify the formid to load that slimmed-down personal task form instead of the regular one.