memtest is self booting. Switching the ram to another computer is not a correct way to check it. Use the program memtest as suggested.

to slave a drive to another computer is to remove it from the computer it is in and change the jumper on the back of the drive to "slave" and install it on the same cable as the other computers HDD in the middle connection plug. Depending on the setup of the other computer...you may have to switch the jumper settings to "master with slave" That is for a computer using a flat IEDE cable.

If the drives are SATA then just plug them in and it will automatically determine the slave drive.

Or you can use an usb external HDD enclosure. Install the drive in it (much easier)

Once installed and booted just navigate to the slave drive and system properties > advanced tab > system startup and recovery settings > uncheck the box to auto restart after failure. save and exit and shut down the computer. Remove the slave drive....reset the jumper...install it and start the computer. On boot when it finds a problem it will Blue screen and have an error code. copy the code EXACTLY as stated as it is what will tell us what the problem is that is causing the problem.

If that fails then you are left with only 2 possibilities....
Do a system repair using the SECOND repair option from a M$ xp installation disc.
Read the section Windows Installation CD - Repair Current Installation:
Accessing The Different Methods of Repair Available in Windows XP
for complete instructions .

The second possibility is to do a complete reformat and reinstallation of the operating system.

OH yes...one last possibility....send it to a shop