Computer problems - won't start up - black screen with flashing underscore

Okay, to start off, I've been having problems with this computer for quite a while, it's an old dell PC I've had for 5 or more years and at one point the computer stopped working overall. Around 1 year ago I tried starting it up and it worked perfectly, my laptop that was my main computer broke so I had to move to the PC as my main computer. 4 or 5 months ago the computer froze with patchy rainbow coloured bits all over the screen (don't know how else to describe it) then after that it wouldn't start with messages like 'diskette drive 0 seek failure' and 'AMS Bios not installed', it then went to a black screen with a flashing underscore and remained then eventually after 15 minutes just a regular black screen. I then found out a way to start the computer up and played fine for a while, with regular freezes that lasted 5 minutes, this process of starting the computer was pressing 'f12' (Boot Menu) on startup, then the diskette message appeared and I had to press 'f1', then the AMS bios message quickly flashed and went to a blue Boot Menu window, I then pressed enter on a '- SATA-1' with no luck, just a flashing underscore, then I tried '- SATA-0' and it amazingly booted perfectly, I had to do this sata-1 process every time the computer started, I did this for 4 months or so working fine until a freeze remained frozen causing me to shutdown from the power button, I went to do this sata process and it didn't work, just a black screen with a flashing underscore, both sata 0 and 1 failed, I then took the power cords out and took the sliding case off revealing the computer parts and brushed the dust out with a paintbrush, I took out the ram and blew it like an old sega game then put them back in, I put the case on then restarted, the computer turns on, then off, then on and off then works, I tried my f12 process, still didn't work and now I'm here hoping people can help since I don't want to spend hundreds of dollars to repair it. If people read this far I thank you SO much and hope you can help, I know it was very long but I tried to be as descriptive as possible, I will appreciate any possible solutions! Thanks.