If you are not the shell kindda user or a command prompt buff, you can use this small but effective GUI ( (Graphical User Interface) ) tool for applying patch to your Drupal files. I've noticed the various kind of errors esp in modules such as UberCart, Advanced Users and also when your web server is using a PHP 5.3.x version, and applying the "relevant patch" seems to be the only solution.
Download this awesome tool here ==> http://tortoisesvn.net/downloads.html. There are 2 versions, namely the 32-bit or the 64-bit. Download and install the relevant software with respect to your Operating system (32 bit or 64 bit). You might have to restart your computer after the installation.
Here's how you apply the patch using Tortoise SVN:
Before you apply the patch please backup your database and the file system (your drupal folder on the web server) else you might repent later on...
1. Download the relevant patch from Drupal. Click on the patch.
2. A Window will open from where you can select the folder you want to patch. Yes, the folder and not the file, (Tortoise will automatically detect the relevant file that's needed to be patched).
3. Finally go to File and Save.
Update [19/May/2012]: I found a more detailed description on this topic on the Tortoise SVN website. Here's the link: http://tortoisesvn.net/docs/release/TortoiseSVN_en/tsvn-dug-patch.html