Andrew, have you followed the information provided by Microsoft for Vista and using Startup Repair?
Have you tried doing a System Restore to put the system back to when it was last working fine - this should help if the problem is caused by a bad Windows Update fix or similar system change?
How much memory do you have on your Acer Aspire computer - Vista needs at least 2GB to run reasonably well, more if available subject to the limit of 3.5GB for 32bit operating systems.
Even though you are running Norton 360 security software - it is still possible that you may have a virus infection - all security software is only as good as the decisions made by the computer user - click on a 'bad' link on a web page and answer a security message wrongly and all bets are off!
Download and burn a copy of the free AVG Rescue CD
and boot your computer from this to check that you have no virus infection(s).
What is the performance like when started in Safe Mode? If it is back to how you expect, then again it suggests that something recently installed has changed things - try disabling any Startup programs - you can use the free CCleaner utility from Piriform
to help you do this by going to the Tools page then selecting Startup which will show a list of what programs are being started each boot - you can disable any of these by using the Disable button or with the mouse on the entry.