Following a number of instances of re-installing Solidworks I thought that it may be a good idea to detail the procedure that I follow each time to ensure a good stable install.
Backup your system. It goes without saying that this is critical in case it all goes wrong.
I use Acronis True Image to take a Disk image of the machine before I do this.
Also make backup copies of all Solidworks Parts, Assemblies, Drawings, Templates, Library Features, Toolbox Data, etc. before you begin. Again, I would recommend making an image of the system to make sure you can access anything you need from the old system.
2. Local Admin and Anti Virus
Log on to the computer using a local admin account. Permissions can cause issues so I would always recommend this.
Also, disable any Anti Virus Software.
Go To Add/Remove programs and Uninstall Solidworks and all other Solidworks components (Edrawings, dxf editor, etc.).
4. Reg Cleanup
Go to Start Menu > Run > type regedit to load the registry editor.
Back up the registry.
Go to HKEY_LOCAL_MACHINE > SOFTWARE and delete the Solidworks Folder.
Do the same in the HKEY_CURRENT_USER > SOFTWARE.
Download and run a registry cleaner such as CCleaner or similar.
5. System Cleanup
Go To C:\Program Files and C:\Program Files\Common Files. If there are any Solidworks folders in these directories delete them. (If running 64bit, check all Program File Folders).
Go to START and search for files and folders called 'Solidworks'; if any are found, delete them.
Empty your recycle bin.
Log on as local admin and disable any antivirus software.
Check for updates and install them.
Copy back all of your custom files parts and the like.
There are always issues regarding your custom options, so do make sure that you back up all of these, as well as your parts, assemblies and drawings. As I stated at the beginning, it is critical that you perform a backup of you system prior to attempting this as there is a chance that it can result in loss of data.
There is also the issue that you cannot save Solidworks parts, etc. as previous versions, so if you are upgrading to a newer version of Solidworks, be sure that you test the system thoroughly before opening any of your parts, etc., as there is no going back.