I maintain a business fleet of about 85 systems, and three dozen of them are variants of
this one that I built. A self-built computer can be as reliable as an HP/Gateway/Dell/EMachine, yes. My 2¢ worth:
1) don't buy cheap stuff. No cut-rate RAM, no freebie power supplies that came with a cheap ricer case, no el-cheapo motherboards.
2) get an APC uninterruptible power supply with automatic voltage regulation and auto-shutdown software. This stuff all runs on electricity, so... yeah
3) equip the system with a spare hard drive and schedule Microsoft Backup to back up your important stuff (heck, and your schoolwork too!

) to the second drive every day while you're at class, or another time when the computer will be on. You can "go back in time" to recover a file you blank-saved or deleted, or if you just went the wrong direction with a school assignment and want to go back to a previous version.
4) Secure the system as well as you possibly can. It would be smart to always have a router between your computer and all other computers, if you ask me, and the other obvious stuff like patching and properly-configured, current-generation antivirus software.
more security ideas here and on the following pages. Set Windows up so the only account a visitor/roommate could possibly log onto is a Limited-class account, or else none at all. Have the Admin accounts password-protected.
5) Get WinXP Professional so you can use Microsoft Backup and join domains and stuff. Don't get WinXP Home.
I would lean towards having your self-built system on the basis that if you know how to build it, then you know how to
rebuild it. Ask a soldier in the field if he calls his rifle manufacturer for help when his rifle won't fire

Uhhhh... NO. If your dad overrides you, then still set your Happy Meal? computer up tight, protect it with
a router, and plan a backup/recovery strategy with an external drive or a second drive.