If you are unable to launch or login to Curse Client v5, it is often due to a corrupt or outdated Microsoft.Net installation. Before trying these steps please try this guide first.
Delete your 2.0 folder
From the Windows Start menu > Accessories choose Run (Or Windows Key + R)
Enter the following, depending on your version of Windows, then click the OK button:
On Windows XP: %userprofile%\Local Settings\Apps (e.g. C:\Documents & Settings\Username\Local Settings\Apps)
On Windows VISTA / 7: %userprofile%\AppData\Local\Apps (e.g. C:\users\Username\AppData\Local\Apps)
On Windows 8: %appdata%\Local\Apps (e.g. C:\users\Username\AppData\Local\Apps)
Delete the 2.0 folder
Run Curse Client v5
Note: You may want to also clear out your computers Temporary files as well. You can use a program such as CCleaner to help you with the.
If this also doesn't work please proceed with the following.
How to Fix a Missing or Corrupt .Net Install
Curse Client v5 requires Microsoft.Net Framework 3.5 Service Pack 1. Ideally, this is automatically installed for you by the Curse Client installer, if it is missing. In some rare cases, this process can fail, and .Net must be installed manually.