The .NET updates are known to fail and how to fix is widely discussed. The quick fix is to uninstall the .NET that failed and reinstall it. However if you don't use .NET apps, why install it?
"Is it safe to download/install this update without any harm to my system?"
Sadly my answer must be no. Given today's malware we can't state that any changes are absolutely safe. This is why so much is written about backup and keeping the malware off the machines.
But has these updates bothered any of the machines at the office here? My answer is no. We have yet to see any issue. But in the real world and these forums you find folk that did have trouble.
Bob
I have a laptop that came with Windows Vista Home Premium Edition and have set the Windows Update
to "Check for updates but let me choose whether to download and install them."
Usually there is a number of "important updates ... available" and a number of "optional update ...
available"
Sometimes out of the important available updates, a few are unselected and are left out during
installation. For instance, at the last update, following were unselected:
1. Security Update for Microsoft .NET Framework 3.5 SP1 on Windows XP, Windows Server 2003, Windows
Vista, Windows Server 2008 x86 (KB2416473)
Download size: 1.4 MB Published: 30/09/2010
2. Security Update for Microsoft .NET Framework 3.5 SP1, Windows Vista SP2, and Windows Server 2008
SP2 x86 (KB2416470)
Download size: 3.7 MB Published: 30/09/2010
My problem is whether they are really important to download and install them since the system
itself could not select them among the others at the time of update.
The "optional available update is:
Windows PowerShell 2.0 and WinRM 2.0 for Windows Vista (KB968930)
Download size: 32.4 MB Published: 22/06/2010
Is it safe to download/install this update without any harm to my system?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic