Monitors, LCDs, CRTs...

On old CRT monitors you may notice a flicker, that is caused by the refresh rate being set too low, using the Control Panel in Windows you can adjust the refresh rate from 60 Hz to a minimum of 75 Hz, check to see what your monitor can support in terms of resolution and refresh rate. This does not apply to newer LCD or Plasma screens, it will be 50 or 60 Hz regardless and you can't change it!
If the fonts are not clear or graphics are a little fuzzy on a new LCD / Plasma screen check you are using the highest resolution the monitor supports. If the fonts become too small there are options to adjust font size by theme or DPI measurement.

Security

Clear up temporary files, old emails, cookies frequently there is software to automate these processes. It is recommended to use them with caution.
Any password should be alphanumeric, using both cases, symbols or numbers when possible. Examples of bad passwords are mydogiscallednelly, froggy1 examples of good passwords MyDogiscAlledN3llY or even better tH1sP@s5woRdi$5ecur3. Avoid pet names as the sole password, birthdays, colours, personal information, etc. Try to randomise it as above. Remember don't write them down, or use a sticky notes!
Wireless PSK, pass phrases, etc. Apply similar passwords formats as above.
Encrypt documents and files, again using strong encryption, read about the types of encryption and the methods involved first!

Uninstall programs no longer in use
Remove temporary files and empty the recycle bin
Check the disks and defragment monthly
Windows Vista
Windows Vista and maybe XP may appear to be slow to shutdown if the system is set to delete the Paging File on shut down. You can disable this by editing the registry key HKEY_LOCAL_MACHINE then SYSTEM then CurrentControlSet then Control then to SessionManager finally MemoryManagement now look for the option ClearPageFileAtShutdown 1 means its on 0 (zero) means off, make sure it is set to zero.
On Windows Vista you can enable or disable features, this is found in the control panel. I disable the Windows DFS Replication service (although you may need this to keep files synchronized) Printing Services, (clear the box) and Remote Differential Compression. These are normally Vista to Vista computer networking options and can cause slow file transfers or faults with browse SMB shares on Vista. On Linux or UNIX you may need NFS option enabled.
As for the network, I tend to disable IPv6 and QOS, again check if you need these or if not disable them.

Windows Key or Win Key is the one on a standard Windows keyboard on the left hand side between CTRL and ALT the symbol & is not part of the combination but is there to explain that you press one key and another whilst holding down the previous key. Note, press the next key once - do not hold it or you may have opened a lot of windows or pasted too many times!
Win Key & R opens the RUN dialogue
Win Key & D shows the desktop
Win Key & M minimize window
Alt & Tab task switching
Alt & Tab, press the shift key and hold now press the tab key, the task switching goes two ways!
ALT & F4 close program
CTRL & C copy
CTRL & V paste
CTRL & X cut
CTRL & ALT & DEL task manager

IPCONFIG - displays basic IP information including IP address and gateway address
NETSH INT RESET ALL - resets interfaces
NET STOP DHCP - stop DHCP service
NET START DHCP - start DHCP service

devmgmt.msc - device manager
ncpa.cpl - network connections
appwiz.cpl - add / remove programs
notepad - you guessed it - opens the text editor
iexplore - internet explorer (in case your shortcut is not working or something)
firefox - launches firefox if you have it installed
explorer - opens the file browser
If your computer has no task bar or icons on the desktop and should have, try opening task manager CTRL & ALT & DEL and click File, New Task and type in explorer it should open the GUI part of the OS, if not and you need to restart the task manager has the options