After installing the R8 update, you may be unable to open QuickBooks or you may see the error:
QuickBooks has stopped working

What you can try now
Intuit recommends 2 solutions for this problem. The first solution may solve your problem, or you may need to try both to resolve the issue. For best results, perform the solutions in the order shown.
Solution 1: Rename the QBWUSER.INI file.
Important: If this issue is caused by hard drive corruption, contact a qualified technician and check your hard drive for errors.
The QBWUSER.INI file stores the locations of your most recently opened company files. This is similar to the way your internet browser tracks which web sites you've recently visited. You see these file locations in the No Company Open window:

A list of recently opened files in the No Company Open Window
Renaming the QBWUSER.INI will temporarily eliminate this history. Print the file to your printer, or make notes if those locations are important to you. Steps are provided below. Once you reopen those files in QuickBooks, the history will be restored.
Windows 7 or Vista
- Open the Windows Start menu, and select Computer.
- From the Organize menu, choose Folder and search options.
- Click the View tab.
- In the Advanced Settings section, under Hidden files and folders selectShow hidden files, folder, and drives.
- Once there is a checkmark next to Show hidden files, folder, and drives, Click Ok.
- In the Computer window, double-click the C:\ drive, and navigate to the following path:
C:\Users\[Your user name]\AppData\Local\Intuit\QuickBooks [year]
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- Right-click on the QBWUSER.INI file, and choose Rename.
- Replace .INI with .INIold and press the Enter key.

- Open QuickBooks.
Windows XP
- Open the Start menu, and select My Computer.
- From the Tools menu, choose Folder Options.
- Click the View tab.
- In the Advanced Settings section, under Hidden files and folders, select Show hidden files and folders.
- Click Ok.
- In the My Computer window, double-click the C:\ drive, and navigate to the following path:
C:\Documents and Settings\[Your user name]\Local Settings\Application Data\Intuit\QuickBooks [year]
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- Right-click the QBWUSER.INI file, and choose Rename.
- Replace .INI with .INIold and press the Enter key.

- Open QuickBooks.
If you still can't open Quickbooks, try the next solution.
*** Before attempting the following solution, be sure UAC is on, then try again, after rebooting. Also try creating a new Admin user in the event that the current Windows User Account may not have full admin access.***
Solution 2: Reinstall QuickBooks using a clean install.
A Clean Install is a more thorough process to replace all program files.
Detailed instructions