First time PC builder

Just a note, when I said I could back up my hard drive onto a CD-RW I meant that, say, every week I could write it onto a CD, but overwrite onto the same CD again and again, the quality may decline after a while, but CD's are cheap as chips these days, in some cases cheaper , the only problem would be I expect it would take a rather long time to back up the drive, and as the computer will be set up in my room i wudnt want it on overnight, due to the noise. But if re-writing onto a CD once a week, or once a month will save me buying a back up hard drive then i reckon its a good idea, just wondering if it'd be possible.

You'll need lots more than one CD and would have to be in attendance to change the CD's after each is written.

Yeh, I get what people have been saying now, I'd sumhow been thinking my hard drive would be 300MB and therefore would fit onto a single CD, crazy I know but ive had a lot on, exam stress and all , now that ive got that unrealistic idea out of me head, ive been thinking of alternatives. The main idea ive got involves having a spare, say 20 or 40GB hard drive and just backing up essential files, for example game saves, and then if the HD crashed all i'd lose would be school work , or something like that. The only problem is that as the build is going to be incredibly expensive (for me anyway, for anyone who hasnt worked out im not a full time earner, im currently in the GCSE period of Yr 11, earing £5 a week pocket money , hopin 2 get a job over summer hols though, the entire project relies on it) i wudn't be able to buy a new back up hard drive, and therefore my only alternatives would be to take either a 40GB hard drive from an old Time (read crap) computer, or take the 20GB hard drive from my current PC (read newer but crapper), these would be incredibly slow, not really a problem as they would only be back up drives, but I'd like to know whether they would slow down the PC for being connected. Again thanks for all your help, no I keep saying it but I dont want my first build to be a disastor and your advice should help me steer clear of that situation, also does anyone know where to buy an anti-static wrist band, I live in the UK if that affects anything but Ive heard there worth buying.