It could be the OS malfunctioning, or some malware protected by a rootkit.
Take a look at the Event Viewer, and consider the possibility of a good session of rootkit hunting. (Rootkits amuse me, they are more challenging than other malware
![](/static/smilies/smile.gif)
)
I cannot stress enough the importance of being careful when using software that can have a deep impact on a working system, such as an antirootkit. Although not certainly favourites of mine, BlackLight Beta, Sophos Antirootkit and AVG Antirootkit may help, especially if you don't want to fiddle with the best ones, which need a better OS knowledge. Before you delete or disactivate something get some information about it. If you find that you don't have important permissions (such as SeDebug) while having admin rights then it's definitely malware.
Using a Restore point to go back when things were working may help too, but this isn't generally a good idea when you suspect malware is involved.
Check also - with Sysinternals Autoruns - the presence of suspect Winsock Providers, along with different suspect stuff.
Otherwise do what laymen advise: reinstall. For me it's seldom a good idea, but it just depends on your willingness to understand and solve a problem. I do it only when I find a damage I deem too extensive to repair (such as lots of lost permissions and deleted files), so a reinstall saves time.