pc is only 15months old and last few weeks it keeps crashing!!! Phoned up helpline and the guy said its maybe the fan? so I downloaded speedfan and I'm gonna watch that adn see if temps run high.

If they do and its the cpu fan, do I have to get any specific type of fan? and can it be easily replaced??

Im kinda new to this, but my suggestion would be make sure your heat sink is plugged in the right spot, and is snug on your mobo. Also make sure all your fans are hooked into your PSU.

Open up the left side of your case to expose it's interior. Put the machine on and confirm that all your fans are in fact spinning. If they are then the likely problem is software related. You probably have too many apps running at the same time and/or your computer is infested with malware. First run msconfig and look at the startup tab. The only things you need checked in there are your firewall and anti-virus programs. Then run a scan on your computer for malware with spybot s&d as well as adaware (both free) and clean house. ALSO, while the case is open, get a can of compressed air and clean the inards of your computer paying special attention to all the fans. If this does not work then you may need to reseat your cpu. This is a simple matter of removing the heatsink/fan combo and cleaning the surface of both the cpu and heatsink. Reapply thermal paste and put it back together. This also applies if the cpu fan is not spinning. In that case you will need to get a new fan rated specifically for your cpu (which you did not mention so I can't explain further.)
This should lead you in the right direction and if you need further explaining then post back with more questions. We'll be glad to guide you.

ASUS P5GD1
P-4 3.4 SOCKET 775
1G Corsaire PC400
ABIT RX600SE 256MB PCI-E

crashing while doing cpu-intensive programs? Or throttling down?

What are the BSOD (blue screen) messages? Go into event viewer (control panel->administrative tools->event viewer), and look through the logs to find the message associated with the crash.

what is malware happeee?? I'm totally new to being able to solve my own pc problems so I kinda need a dummy proof guide to things. Can you break down what I've got to do for me please? really sorry but I'm totally lost.... must be just cos I'm female!!!

Malware is any software that installs itself on your computer without your knowledge. It often "piggy backs" on another legitimate program you installed. Anyways..........
Click start and then click run and in the command line type 'msconfig'. This will open up a series of tabs with info. Click the startup tab and uncheck everything in there EXCEPT your antivirus and firewall programs which you can identify by either 'symantec' or 'mcafee' being in the name. Click apply and then OK and then click OK to reboot. On startup it will tell you you are using selective startup blah blah blah. Put a checkmark where it says do not show again and click OK.
Now, open up your browser and go to the tools tab at the top. Click on Internet Options and then click delete cookies and delete files. Also click on delete history and 'X' out of it. Now google 'spybot s&d' and download it and install. Open it up and click OK at all the warnings. Keep hitting next on the box that opens up in the upper left till it says 'start using program'. It will then run. Click updates. Download and install all the updates. Now run Spybot by clicking check for problems. It will take a little while to finish but when it does it will give you an idea of what malware is on your comp. Click on Fix and let it work. Click yes if it requires to start on reboot and do the same when it's done.
Now do the same as above with a program called Ad-Aware from Lavalys. It works basically the same way and it will hopefully catch things spybot didn't. I don't think you can possibly have too many programs to check your computer for malware. A computer that is malware-free will run better and smoother and it won't crash (unless of course it's hardware related.)
And don't fret, all of us were beginners once. Over time you will learn more and actually help others too. It isn't because you're female. If you're a blonde however............

ASUS P5GD1
P-4 3.4 SOCKET 775
1G Corsaire PC400
ABIT RX600SE 256MB PCI-E