When you've tried everything and you're just about to give up, format and reinstall, hang on, try this first.
Log in as Administrator then go to Control Panel > User Accounts and Create a new account. Make it an Administrator or Limited account according to your preference.
Open Windows Explorer > Tools menu > Folder Options > View tab enable Show hidden files and folders.
In the Folder pane on the left, expand C:\documents and Settings and highlight the name of the your old, slow account. In the File pane on the right select all the files and folders, right click and choose Cut from the context menu.
In the Folder pane on the left, click on the name of your shiny bright new account then in the right hand pane right click and choose Paste on the menu.
When your documents have finished moving log out of the Administrator account and log into your new account, which should now run wonderfully.
When you've confirmed all is well, log in again as Administrator, return to Control Panel > User Accounts and Delete the old account.
A further tip:
Keep your Documents directory on a separate partition or drive. When you do the above and create a new User Account, don't copy the contents of the old account to the new one as described above; do this.
On the Desktop, right click on My Documents, and choose Properties on the context menu. Click on Move. Browse to your Documents folder on the other drive or partition and click ok.
Confirm the move when asked (in fact there are no docs to move, only the default My Pictures directory etc), the My Documents directory should now point to your Documents as before.
Keeping your Documents folder separate also makes it much easier for all those regular backups I know you all do