I keep getting disk boot failure when I restart my computer.?

I have Windows XP on my computer. In order to remove a trojan on my system, I attempted to put my computer in safe mode. After doing so, all that came up was white text on a black screen. I waited for a while, but nothing else came up. So, I turned off the computer. After turning it back on, the computer said "disk boot failure." It also instructed me to insert the system disk, but my computer never came with the Xp disk. It was already loaded on my computer. To alleviate this, I purchased a Windows Vista Upgrade disk, but it's not letting me load it since it's an upgrade. Please, help me. I am at my wit's end. Also, I am very computer illiterate, so in any instructions please be as basic as possible. Thank you.