Pc acting very strange

Hi guys,
On my sisters pc, I press the button to turn on but the only thing that starts is the processor and the graphic card fans spins up, the HD doesnt or the dvd drive wont open and the screen doesnt come on either just acts like its in standby (by that I mean the light flashes).
its very strange and I cant sort it, Ive removed all cables and plugged back in, ive unplugged dvd drive and tried, also unplugging HD and graphics card.
Something is either failing or failed.

If the hard drive wasn't working as stated, you wouldn't see any activity...

yeh I cant see anything the monitor doesnt come on at all, the HD doesnt even spin up.

Please fill out your system specifications because you can't get expert help without letting people know what your hardware is... :)

The two most likely suspects are the power supply and motherboard. Even though you are getting partial power, that doesn't mean that all of the rails are active. You need to check the output on each, with a DVM. If the PS is good, disconnect everything from the motherboard, except a single stick of ram and video card, then short across he power pins on the motherboard to see if you can get a BIOS screen on the monitor. If not, then the motherboard is faulty. If you do get a display, some component is throwing a monkey wrench into the works, so reconnect them one at a time until the display fails, thus determining the culprit.

EDIT: You don't need the HD to display a BIOS screen.

After checking with one stick of ram, as described above, swap sticks and check again. You might also use a different RAM slot.

When did this PC last work well? Who has been operating it since that date and what might they have done to it--installing questionable programs, navigating to dodgy web sites, etc?

Sorry about that was in a rush, usualy I can sort problems out myself but this one escapes me and plus I dont want to spend the rest of my holiday fixing a smegging pc lol, so thats why I came here the product information is here, not much has been changed from that listing apart from HD and GC:
Product Specifications Compaq Presario SR1219UK Desktop PC - HP technical support (Ireland - English)

its model number:sr1119uk if you need to search.

When did this PC last work well? Who has been operating it since that date and what might they have done to it--installing questionable programs, navigating to dodgy web sites, etc?

my sister uses it for her uni work, ive told her not to use limewire etc

The two most likely suspects are the power supply and motherboard. Even though you are getting partial power, that doesn't mean that all of the rails are active. You need to check the output on each, with a DVM. If the PS is good, disconnect everything from the motherboard, except a single stick of ram and video card, then short across he power pins on the motherboard to see if you can get a BIOS screen on the monitor. If not, then the motherboard is faulty. If you do get a display, some component is throwing a monkey wrench into the works, so reconnect them one at a time until the display fails, thus determining the culprit.

EDIT: You don't need the HD to display a BIOS screen.

After checking with one stick of ram, as described above, swap sticks and check again. You might also use a different RAM slot.

will try that now, cheers

tried all that, the CPU fan spins at what seems to be 100%, removed mem, unplugged everything, screen doesnt come on at all even removed the CMOS battery.