Magento module installation always has a specific order of things that must be done for successful module install.
Only install one module at a time.
The following things must be done:
If you have the compiler enabled, disable the compiler either by the backend utility or the shell script shell/compiler.php
. If you've been running up to this point with it disabled, leave it that way as it has been shut off for a really good reason.
Disable all caching in the cache management screen and then clear the cache either by clicking the button or manually deleting all subfolders in var/cache/
Log out of the Magento backend.
Install the module
Log into the Magento backend, the installer scripts will run, the system will seem to hang momentarily at login.
Once logged in, log back out and then back in to reload all the admin panels that might be associated with the module
Clear the cache again, even though the cache is disabled, there is a lot of Zend and low level stuff that is cached despite the settings.
Run Magento and test that your front end and back end are running without problems
Enable all the cache items you disabled in Cache Maintenance and check that Magento is still running well with just caching turned on.
ONLY AFTER proving the system is running properly, IF YOU WISH to enable the compiler a) Recompile the code base and then b) reenable the compiler
If the system will not run with the compiler enabled, disable it with the shell/compiler.php
script or search elsewhere for manual file edit. Contact your module developer to see if it will run with the compiler, otherwise leave the compiler disabled, it has become rather irrelevant from 1.7.x.x forward as a speedup and usually becomes a source of problems if you're not running plain vanilla Magento.