From http://wiki.vtiger.com/archives

Installation & Uninstallation Issues

• Check the vtiger & outlook plugin version`s.
• Un-install the existing plugin (Add/Remove Programs). If it does not uninstall follow the below steps.
• Check if any other add-ons are installed for outlook, if installed check their dll`s. Ex : supervtiger plugin
• Locate vtiger Plugin`s registry entries.

 Found in the below mentioned location
regedit → HKEY_LOCAL_MACHINE SOFTWARE Microsoft → Office → Outlook →Addins → vtigeroladdin.service

• Delete the entries from registry.
• Even after the above steps it appears in Outlook, follow the below step.
• Place to manually delete the vtiger-Outlook plugin installation entries :

 C:\ProgramFiles\Installshield Information\{93ED8D1A-0552-4AB1-A0E6-4F8C4F9FE295}
Remove the folder.

• Try installation again.

Log In issue with registering DLL files

When working with vtiger CRM Outlook plug-in if you notice the Popup like "Pocket Soap dll not registered" or similar, its an indication that few of the dependent DLL files have not registered as expected.

You can register the DLL or OCX files manually as described below to have the plug-in work in expected manner.

(It is recommend the following operations be performed by administrator)

Open the command prompt and execute:

regsvr32 "C:\Program Files\vtigerCRM\Outlook\resources\support_dlls\pocketHTTP.dll"

regsvr32 "C:\Program Files\vtigerCRM\Outlook\resources\support_dlls\psDime.dll"

regsvr32 "C:\Program Files\vtigerCRM\Outlook\resources\support_dlls\pSOAP32.dll"

regsvr32 "C:\Program Files\vtigerCRM\Outlook\resources\support_dlls\psProxy.dll"

regsvr32 "C:\Program Files\vtigerCRM\Outlook\resources\support_dlls\MSHFLXGD.OCX"

regsvr32 "C:\Program Files\vtigerCRM\Outlook\resources\support_dlls\TABCTL32.OCX"

regsvr32 "C:\Program Files\vtigerCRM\Outlook\resources\support_dlls\vbalProgBar6.ocx"

C:\Program Files\vtigerCRM refers to Outlook plug-in installation directory, it could be C:\Program Files(x86)\vtigerCRM on some machines

Instead of running the above commands manually, you can run the batch file from the following link and register the DLL`s.
http://vtiger.com/products/crm/outlook/outlook_registry.zip

Can't Initialize the log file...

At runtime vtiger CRM Outlook Plug-in attempts to create data and log files under Outlook plug-in installation directory (C:\Program Files\vtigerCRM\Outlook).
Make sure suitable write permissions is enabled for the user in the directory.

Invalid Return Value from vtigerCRM

This error will crop up due to the following reasons :
1. If there is an extra blank line at the end of config.inc.php file before the PHP closing tag.
2. If there are accent characters in any of the records that are being synced.
3. If there are special characters like <,>,/ in any field.

Error while syncing contacts from Outlook

The above mentioned error will be seen if Lastname field is blank in outlook contacts.
Since lastname is a mandatory field in Contacts module of vtigerCRM, it cannot be blank.

Unable to connect to vtigerCRM Server

This error will appear due to following reasons :
1. If firewall is enabled on the server where vtiger is installed.
2. Restricted HTTP access.
3. If the login credentials provided while logging in through plugin are incorrect.

HTML Tags stripped on Associating an Email

After associating an email with a Contact in vtigerCRM, if you find HTML tags in the content. And if entire email content appears without original formatting, please check your libxml2 version.

This is a bug with libxml2 2.7.2 version.

You can refer to the following URL for more details : http://josephscott.org/code/wordpress/plugin-libxml2-fix/