Either buy from Dell, or if you're lucky and have a mate who genuinely knows what they're doing, ask if they'll help build one. Its not as easy as it looks, you have to be careful to protect the chip from any sources of static for starters. You have to know which components go together when you buy them.
Are there any courses near you on 'build your pwn pc'? Thats another good way to do it.
But bottom line is that the big manufacturers like Dell gat massive discounts for buying on bulk, and the price at which you can buy your components you're not going to save much money. With Dell you get a warranty; with self build if you fry your motherboard you have to go buy a new one.
You won't get a pc that plays games or edits video on your budget, but you'll be able to surf the web and does other stuff with not too much problem. Just don't get a Celeron!
You might be able to save a few quid by buying a CRT monitor instead of a flatscreen. And you also need a decent desk and chair which bumps the price up.
Don't spend any money on security software or an office suite, you can get all that as legal freeware. Here's a few usefull addresses;
You need to install one firewall.
The Windows firewall doesn't stop malware on your pc from dialling out. Both these firewalls are free
for personal use and stop unwanted incoming and outgoing traffic.
http://www.zonelabs.com Zone Alarm firewall
http://www.comodogroup.com/ - Comodo security suite.
Shields Up security site;
https://www.grc.com - check your firewall works
http://www.grc.com/optout.htm - grc anti spyware program and info.
http://www.mozilla.com/ Firefox web browser
http://www.opera.com/ Opera web browser
Instal both these web browsers, turn off javascript on one and use that browser for vising unknown
websites. They don't support Active X so thats one less way for problems to get on your pc.
You need to install one anti virus;
http://www.grisoft.com/doc/1 AVG anti virus or
http://www.avast.com - Avast Antivirus
You need all three anti spyware programs;
http://www.ewido.net - AVG(Ewido) anti trojan.
http://www.lavasoft.com - Ad-Aware SE Personal anti adware
http://www.safer-networking.org - Spybot Search and Destroy anti spyware
And these are useful utilities;
http://spywarewarrior.com/rogue_anti-spyware.htm - The Spyware Warrior site identifies fake anti
spyware programs.
http://www.ccleaner.com Cr*pcleaner Registry cleaner and privacy tool
http://www.mailwasher.net/ -spam filter
Winpatrol is a handy utility that tracks cookies on your pc and stops unwanted changes to your
startup folder.
http://www.winpatrol.com
Spywareblaster works with Internet Explorer and Firefox. It won't uninstall spyware that is already on
your pc, but it will prevent hijacks and other malicious changes to your browser.
http://www.javacoolsoftware.com
http://www.siteadvisor.com -Site Advisor warns about malware on sites you are about to visit.
Remember to update and run all of you anti malware programs at least once a week.
http://www.sys-shield.com - AbsoluteShield File Shredder
http://heidi.ie/eraser - Eraser file shredder
http://www.openoffice.org - freeware office suite
http://audacity.sourceforge.net/ -Audacity music tool.
http://www.ejay.com - music making program
http://www.eecs.wsu.edu/paint.net/ -Paint.net Paint with layers.
http://www.irfanview.com - useful graphics tool
http://www.download.com/Deep-Paint/3000-2191_4-10398243.html - Deep Paint
http://www.gimp.org - Like Photoshop 7
http://www.inkscape.org - vector image tool
http://www.wantedfonts.com - more fonts than you can shake a stick at