#1: A process that's loading down your CPU:
It could be a sub-process or application that's running in the background
and taking all the CPU resources, which could be the cause of your PC
running slow.
To find and display what could be the problem try Process Explorer:
http://technet.microsoft.com/en-us/sysinternals/bb896653.aspx
Note: Once you have Process Explorer installed and running:
In the taskbar select View and check 'Show Process Tree' and
'Show Lower Pane' options.
Then expand the process named 'Explorer' (click on the + sign)
In the column on the left named 'CPU', look for any high CPU usage.
Next click on the CPU column to sort the processes by %CPU usage
(Highest to Lowest).
Move the mouse cursor over any process, you should see a popup with some
detailed info.
Then mouse over the process that's using most or all the CPU %.
Then click on that process to highlight it,
Now that it's highlighted, right click and from the options listed select:
'Search Online'
This should display what out there on the web about that process.
You can also double click on any process to open up a more detailed
'Properties' window.
Note: some entries like Explorer, System/Services, and
Svchost entries may need to be expanded to show the detail (sub processes),
in this case click on the + located to the left of the entry.
An alternate method using Process Explorer is to double click
on the Graph just below the Menu bar.
This will open the 'System Information' window, which has a larger display
of all three graphs.
Move your mouse over any spike in the CPU Usage graph to see what
process/application or service was the cause of the spike.
#2: Stuff that loads during boot or logon and then is always running in the
background:
If you want to list and explore what may be the cause then:
Try Autoruns from the MS Windows SysInternals site:
http://www.microsoft.com/technet/sysinternals/ProcessesAndThreads/Autoruns.mspx
AutoRuns will show/list all apps/etc. that load/run when you first boot
(Boot Execute tab),
when you logon (Logon tab) and other programs that load
(grouped by labeled tabs) for easy viewing.
It also provides the ability to selectively allows you to stop any program
(use with care) that you don't want to load.
You can undo any changes you have made.
Note: To get additional details on an item in the list you may need to
highlight the item (right click) and use the 'Search Online' option to get
the details, especially useful for the more obscure items in the list.
#3: Viruses
Viruses and malware can also cause your PC to slowdown or malfunction.
Malke has an excellent set of instructions on what to do:
http://www.elephantboycomputers.com/page2.html#Removing_Malware