I ended up formatting the drive and installing a fresh version of Windows. It seems that this resolved the stuttering issue.
I bought the machine from Overclockers UK.
By the way, after installing hundreds of Windows updates, one got stuck. The details of the update are below. Any idea how to force this to go through? Thanks a lot.
Security Update for Microsoft .NET Framework 3.5.1 on Windows 7 and Windows Server 2008 R2 SP1 for x64-based Systems (KB2656373)
Installation date: 15/07/2012 20:46
Installation status: Failed
Error details: Code 80073712
Update type: Important
A security issue has been identified that could allow an authenticated remote attacker to compromise your system and gain control over it. You can help protect your system by installing this update from Microsoft. After you install this update, you may have to restart your system.
More information: MS12-025: Description of the security update for the .NET Framework 3.5.1 on Windows 7 Service Pack 1 and Windows Server 2008 R2 Service Pack 1: April 10, 2012
Help and Support: Microsoft Support