is leaving my pc running all the time a good idea?

Windows Vista ClockPCNS gets this question asked a lot.  No doubt, as you explore the Web for answers, everyone's going to have a different opinion. The answer depends on what have you been doing?

Scenario:  I have a new PC.
Answer: Shut it off at night, or when you won't be using it in the next 6 or more hours. It's important to power cycle the PC during the first year.  This is because it's under warranty and if or when problems show up you want the computer to malfunction while it's under warranty.  Turning off the power saves on your utility bill, it attracts less dust to the insides, and extends the life of your monitor (flat screen or crt).

Scenario:  I have left my pc running 24 x 7 for the past few years.
Answer:  You should leave it running 24 x 7.  Changing operating habits now may have adverse affects, because your PC has been conditioned to run all the time.  The odds of system board failure are much higher, and since your system is no longer in warranty, it will be more expensive to replace.  An alternative may to to allow the computer to go into 'Standby' mode which keeps portions of the system board hot.

In any event it's a good idea to invest in a battery backup, to prevent rolling thunderstorms and power interruptions from taking your PC out of commission, regardless of your operating schedule. 

Sidebar: How green is my PC?  In case you were wondering, I have a middle of the road Pentium 4 2.6 Ghz Computer, with 2 gigs ram, 80 gig hard drive.  According to my Kill A Watt power meter, it consumes 102 watts on normal power on, monitor not included.  Based on 24/7 operation, you are paying:

102 Watts x 24/Hrs a Day x 30.5 days = 74,460 total Watt Hours

74,460/1000 = 74.46 Kilowatt Hours

74.46 Kilowatt Hours * 0.16 per kilowatt Hour = $11.91 per month.

Yearly Cost = $11.91 * 12 = $142.96

Related: Will switching to a Flat Screen save power?