Hello and welcome to bleepingcomputer.
First thing you should do with this computer is get a can of compressed air and a toothbrush, use both to clean out the computer case of all signs of dust accumulation.
Use the toothbrush to clean off all cooling fan blades or stubborn stuck on dust from all of the interior hardware.
Make sure you use proper precautions against electro-static build up damage, ground yourself to the computer case before and during the cleaning process.DO NOT
allow the cooling fans to spin from the force of the compressed air, or for safety reasons, disconnect the cooling fans from the motherboard first.
Your hard drive is most likely failing, by either being affected by too much heat exposure or the drives controller board is failing.
You should try using the proper disk utility and allow it to scan the drive for read/write errors.
Since you have a SeaGate hard drive, I recommend using SeaTools for DOS, make a bootable disk and boot to it. Do not run it from Windows!! This utility is meant to be burned to a bootable disk and booted to.
Here is the link: http://www.seagate.com/ww/v/index.jsp?locale=en-US&name=seatooldreg&vgnextoid=480bd20cacdec010VgnVCM100000dd04090aRCRD
This version listed here can be run from within Windows: http://www.seagate.com/ww/v/index.jsp?locale=en-US&name=seatools-win&vgnextoid=552bd20cacdec010VgnVCM100000dd04090aRCRD
Run the long tests and see what the results are.
As for slow drives, there are a few factors involved here that can cause this.
One is what your UDMA settings are set to, UDMA 5 and PIO 0 is best, some drives resort to PIO 5 and UDMA 0, which is very, very slow!
Make sure your swap file is large enough to handle the load if virtual memory is needed.
Make sure your hard drive has been de-fragmented and cleaned up using Windows built in utilities, disk clean up and disk de-frag.
Make sure your not running low on physical memory, which causes more hard drive thrashing and the use of your swap file more often.
2 GB of memory is not much when running OSes such as Windows XP through Windows 7, adding another GB of RAM would certainly help in decreasing any hard drive thrashing.
Hopefully my suggestions get you back on track, but to be truthful, I believe your seagate hard drive is failing.