my hard disk temperature shows 51 celsius,is this too high?

Yes it will die soon. Be sure to pray for an afterlife of your hdd.

The temperature for CPU / HDD / LCD isn't going to die unless it hits 71 degrees Celsius. However, 51 is starting to get bad. It does tell you that there's a problem.

If you keep your computer in a room that gets hot during the day, don't.
Make sure all of the fans are working.
Back up all of your computer documents, just in case.
And be ready to replace your HD.

also dust out the inside of your computer regularly. the grime buildup can render the fans practically useless

A good way to see how your hard disk temperature compares to others of its type is to install speedfan (google it, made by the same people who made mother board monitor I think), and then click on the SMART tab and click on "Perform an in-depth analysis of this hard disk." It will open up a web page that shows a lot of SMART statistics as well as how they compare to others submitted in the past.

As sergeantedward has said above, you should start backing up regularly.

Is this computer a laptop/notebook or a desktop, because if it is a desktop, then you should either dust it yourself or get someone knowledgeable with computers to do it. You can also contemplate upgrading your cooling system. However, since you mentioned the Intel 945g which is typically found in laptops, then I would suggest getting some external cooling help. I am assuming that other parts of your laptop are also hot. My recommendation is either this, a pad which you can place under your laptop and provides cooling without the use of a fan.
http://www.thermaltake.com/product/Cooler/NBcooling/r15on01/r15on01.asp
If you need even more powerful stuff, then use a fan-system. Just google "laptop cooler" and you should get a lot. They all work pretty well.