A few weeks ago, i tried to install a 320 GB Seagate hard drive. After i installed it, my computer began to restart randomly. I decided it would be a good idea to open my computer, and yank out the hard drive. (I don't know as much about computers as I thought I did.) After that, the computer wouldnt even get to windows. I finally figured it out, moved the jumper, and got it to boot up windows. It still restarts, though. I've run a virus scan and an adware/spyware scan. Tonight I'm going to do the updates that I should have done a while ago, including updating my virus scanner. If that doesn't work, the only other thing i can think of is replacing the Hard Drive cable... maybe I bent or broke something when I was installing the 2nd Hard Drive? I'd love some input as to what could be the problem.

Other helpful info:
-I have 4 fans in my computer, so it's not overheating.
-The Seagate hard drive was a second hard drive, because my 1st one was getting filled up.
-The two hard drives are made by different companies.

does the motherboard support the 320gb hard drive and when you remove the seagate drive does it work porperly

Random restarts always make me think of heat and power issues (4 Fans? Why?)
The fact that you've been tinkering and don't have much experience suggests you should carefully check all your work.
Re-seat everything.
Ramjo has a good question:What happens when you disconnect this drive?

The seagate hard drive has been out of the computer for a while. It's still restarting. As for the 4 fans, one of them is disconnected... and I'm not sure why the guy who built it put in 4 of them.
I was told that i might want to check the hard drive for a bent pin or something... anyone think that could be the problem?

Not a bent pin-you'd know if you bent one, and the HD wouldn't work at all.
Much more likely to be a failing power supply or a heating/cooling issue. Possibly a loose connection or a bad solder joint.

Check in event viewer for any error messages relative to the restarts.

Also right click my computer - properties - Advanced tab - Startup and Recovery Settings - and untick under System Failure - Automatically Restart - OK and apply out of there.

With that unticked an error message should show instead of restarting.

Did you change the jumper of the 2nd hard drive to master or slave and by guessing you installed the 2nd hard disk the system which jumper did you use on it.

I'm with hughv - reboots often are related to power stuff. Seems odd that it would just kick in out of nowhere. Maybe he should pull the CMOS battery see if that resets whatever kicked off when the other HD was installed. I would check cables and memory chips...a bad HDD cable could cause some intermittent weirdness - also seen memory do this. Also, nobody said it yet, could it just be that the drive he has there is failing? Has he run chkdsk or anything? I agree that disabling reboot will be good so he can see what causes the failure. Bad hard drives often cause reboots...but so do fans, bad memory chips, hot cpu's and poorly seated/damaged cables...software! Clean out the case, reseat all the cables and components and call me in the morning...

Good luck!