Freeze or Auto Restart Issue

Hello,
I'm writing here because I saw that another problem got some pretty good answers so I'm hoping that you'll be kind enough to help me as well.
I've been having this problem that started last night when my brother tried to start up the computer. Yesterday I had been browsing The Sims Resource when I suddenly had a tab pop up and I think a window saying "Thank you for downloading" but I hadn't downloaded anything so I was already starting to freak out. The next thing I knew, I had a bunch of Winpc messages saying that I had 24 errors and whatnot. While getting rid of the messages that kept popping up, I headed over to the symantec site and downloaded a free trial of norton antivirus 2009, installed it and had it run a full system scan during which it found 10 issues (at least three of which it said were serious) and fixed them. Or so it said anyway. I stopped getting the winpc messages. After the scan, I played sims for a while and then closed out of it. I went to go change for church. When I came back to the computer, it had a message saying the computer needed to restart for the changes to take effect so I had to restart and went to go do something else. When I came back, it was frozen at the log in screen so I just switched if off manually figuring it wasn't a big problem. When we got back from church, my brother got on the computer. He later came into my room and asked what was wrong with the computer. He said that when he started it, it took twice as long to start up and then when he logged into his account, he went to use the internet and the computer just suddenly restarted. He said that he tried doing that or something else and it restarted every time. No error message or anything, just a sudden restart. I said I'd look at it in the morning and went to bed.

So this morning, I tried turning on the computer. It gave me the screen with options for startup and I just chose to start normally. It started up fast enough and I logged into my account which also came up without any problem. I left the room to ask my brother about details on what happened and told him that it started up fine. He then told me that sometimes it would start up and then when he logged in to his account, it would just freeze. I came back to the computer and, sure enough, it was frozen so I manually shut it off and turned it back on. This time, I logged in and tried clicking on firefox which worked, but as I was waiting for the window to come up, the computer auto restarted. I tried logging into my account and my brother's a few more times having the freezing or auto restart issue before using my laptop (which I am currently on) to look up help. I got several search results saying how to turn off automatic restart so you can see what error is causing it. However, I can't even get to the control panel before the computer freezes or restarts. And in the rare event that the window comes up, it just gives me the searching flashlight which eventually freezes or keeps going until the computer restarts. I no longer get the startup options menu when I turn the computer on or when it restarts so I can't choose to use safe mode or anything. It just buzzes right on by to the account login screen like everything is perfectly fine. I tried shutting it down properly from the login screen but when I turned it back on and tried to log in, it froze at the "Loading your personal settings" screen. I'm really at a loss for what to do at this point. I don't know much about computers to begin with.

As for the computer, I have an Intel Pentium 4 processor. I'm never entirely sure which information to include so I'll just put down all the stuff it says on the silver sticker on the front. That would be:

515*
Intel Pentium 4
Processor
512 MB PC3200
DDR SDRAM memory
160 GB 7200RPM
Serial ATA hard drive
DVD+RW/CD-RW
Double Layer 8X
DVD+R/RW drive with CD writer capabilities
Processor speed2.93GHz 1MB
L2 cache. 533 MHz Front Side Bus