My software cannot be activated because the Product key used to install the software is not valid. It says "please uninstall the software and then reinstall it using a valid Product key. Do not know how to uninstall the software and then to reinstall it again. Please help.

Office Genuine Advantage (OGA) is another anti-piracy initiative by Microsoft on its popular business productivity product – Microsoft Office XP, Office 2003 and Office 2007. Similar to Windows Genuine Advantage (WGA), OGA requires users of the Microsoft Office software to validate their installed copy of Microsoft Office in order to download files from Microsoft Download Center, access updates or hotfixes from Office Update, and receive special offers for genuine users.

OGA is not to be confused with Office Product Activation that is mandatory required to during the installation process in order to use the Office software whereby users enter a serial product key that is validated online over the Internet or by phone by Microsoft to confirm that the product key has not been used on more personal computers than are permitted by the Microsoft Software License Terms. To crack Office XP/2003/2007 activation, you need a key generator that provides volume license key (Guide: How to change Office 2007 product key).
If you’re using a keygen-generated product key or publicly available leaked Office 2007/2003/XP serial number, especially the volume license key (VLK) that requires no activation when used, most of the keys no longer able to pass the OGA genuine validation test anymore with the following error message (Office 2007 as instance):
This computer has failed validation.
Microsoft Office Enterprise 2007: Validation Failed
The product key used to install Office is not valid. It has been identified by Microsoft as a false key used with counterfeit copies of Office.
[0x80180108]
It’s likely that users who failed validation check may continue to use Microsoft Office as per normal and use Microsoft Updates to update critical patches. However, if you want to download add-ins, updates, templates, samples and tutorials, Smart Tags, Language Interface Packs and Software Development Kits (SDK) that require Genuine Office validation, you won’t be able to do so.
To bypass or crack the Microsoft Office 2007 and Microsoft Office 2003 OGA genuine validation, follow these steps:

1. Install and activate Microsoft Office 2007 or other version of Office. If you intend to use the key-gen, you must install Enterprise edition.
2. Download the patched ogacheckcontrol.dll.
3. If you never validate your copy of Office, go to Genuine Microsoft Software web page, and run the Office Validation Assistant by using Internet Explorer (not Firefox as it will use different version of ActiveX component). This is to install the components required for Office validation. If you fail the validation – Office not genuine – simply ignore the message.
4. Copy the cracked ogacheckcontrol.dll to the following location: Windows XP and Windows Vista (32-bit): \Windows\System32\
   Windows XP and Windows Vista (64-bit): \Windows\SysWOW64\
   
5. Attempt to download Office files that require validation again. All downloads should work now.

The OGA Check Control crack works on Office 2007 Enterprise Edition, Office 2007 Blue Edition, Office 2007 Professional Edition, Office 2007 Pro Plus, Office 2007 Standard Edition and all Office 2003 Editions.
If you’re using invalid or blacklisted product key, you may have to change the Office product key (Guide: How to change Office 2007 product key). This is especially so if you’re using Microsoft Office 2007 Blue Edition.
Disclaimer: This patcher is for genuine Office user who having problem to pass genuine validation only. To use Office 2007, you must purchase a license. Alternatively, you can use for free OpenOffice.org, an equally good productivity suite.