The most common cause I find is malware. I use Grif's list of apps to scan and in most cases remove the pests.
Then there are torrent users. That's a whole 'nuther ballgame.
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Something is causing random programs on my computer to use large amounts of memory. After I did some updates, my computer started up and was running really slow so I checked the Task Manager to see there were several random programs such as "dllhost.exe," msdtc.exe," "taskhost.exe" all using up 100-500,000K of memory at a time. Even the command prompt, which I don't have open is using up large amounts of memory. I tried running a system restore to yesterday twice, and it failed both times. Any other advice?