You don't mention what OS you are running, from the posts it seems we are talking Microsoft but is it XP/Vista ?
Do you have any System Restore Points that you can try a restore from ? If you are correct and it is something in the registry then this will recover that issue.
From what you have tried it sounds more like a rebuild of the OS is in order. This is normally a good idea after around 6 months anyway since the machine will get clogged up with software installation, software uninstallations and dll files can get out of sync for different software (i.e. new versions are overwritten, etc..). Its wise to make a base image of the machine to help recovery quickly. Take a look at this post where i explain the process i go through