300-350 dollars? or are you on another currency in Dubai?
If in good old American $, then I always suggest that at the $500 and less market you are better off purchasing a pre-build machine, such as a Dell or HP. You can build your own PC for cheap (see build below), but it is not going to have a real warranty, and you are going to have to cut a LOT of corners on quality.
Below is the build, as I do not know what stores you have available to you I cannot pick any specific parts, but if you follow these guide lines I think you will be pretty happy with whatever you get.
Parts not to skimp out on: Power supply and motherboard
Even if your computer only requires a 350W power supply (which is all you need for a basic PC) then you still want to get a quality name-brand power supply. Sadly I do not know what is available or good in your area (different brands are better in different areas). Absolutely should be 80+ or 80+bronze, not really for the sake of power savings (though there will be some), but for the sake of a quality product. DO NOT use a power supply bundled with a case, or get some junk $15 supply that will die in a year. You should get something in the 250-350W range for an office PC, but if you ever intend to put in an aftermarket GPU then get something in the 450-550W range that has the PCIe power connectors.
For the motherboard, get the cheapest board you can find that will fit a cheap CPU (like a Pentium G, or AMD APU) that has solid capacitors, or at least Japanese made caps, and look for a board with a 5 year warranty... but that is going to be tough with your budget. AMD boards should be AM3 or AM3+, while Intel boards should be LGA1155.
Problem parts: Ram and HDD
Ram is cheap these days, and you only need 2-4GB for an office machine (whatever you buy, be sure to get it in pairs as it will be ~20% faster than a single stick). Ram does not need to be expensive, but you may want to go with Crucial if reliability is of concern... but honestly, ram manufacturers are all hit-and-miss these days, and if you get a good set of ram it will last forever, and if you have a bad set of ram you will know within a month and can exchange it.
Hard drives are kinda like Russian Roulette, even good drives can die very early for no apparent reason, and sometimes cheap drives can last forever. Case and point; I purchased a high end WD hard drive a long time ago, and it died (well sort of died, it kinda still works, but I cannot partition more than 200GB of a 350GB drive... really odd problem) within a year. On the other hand I bought a dirt cheap 1TB Seagate drive (and seagate is not exactly known for quality), and it is 7 years old now (2 years longer than I expected), and I use it every day. In short, do not trust your hard drive to keep your data safe, always keep copies of your important data on CD/DVD, on a flash drive or external HDD, and on a network or cloud because the simple fact of life is that drives die, so be prepared.
Low risk items: cpu, case, keys, mouse, monitor, etc.
The rest of your computer really does not matter what you get no matter how cheap you go. CPUs from AMD and Intel have a less than 1% failure rate and will last forever. Cases can last a very long time so long as they are not physically abused. Keyboards, mice, and monitors rarely die, and are cheap to replace when they do, so they are not a problem.
Keys, Mouse, Speakers, Monitor: Already own $0
Windows: $100 (and do no get a hack copy! especially for work)
Case: Used $0-5, or New $20-30, must be ATX compatible, preferably without a power supply. This is the only part I would trust buying used as you can plainly see any damage.
Power Supply: Again, not sure what is available in your area, but it does not need to be large, just efficient and heavy. 80+ rated, and weight generally means quality, $40-50
CPU: Modern entry level Sempron (AM3) or Celeron (LGA1155) processor $40-65
Motherboard: MSI, ASUS, or Gigabyte $40-60
-This should include network, GPU, and sound
-Must be ATX (mATX, or regular ATX, just not BTX, ITX, so some other odd form factor), DDR3, and either AM3/AM3+ or LGA1155 to match your CPU of choice
Ram: 4GB 2x2GB DDR3 1333 $20 or less
HDD: Any size bigger than 60GB, SATA1, 2 or 3, $75
Optical Drive: Any cheap CD drive $20 or less
Total Budget: $335-360
Again, you can get the same basic computer with a cuter/smaller/quieter case, plus a warranty from any major manufacturer for the same $ or less.
Hey Thanks, Yeah I did Go Yesterday to get the parts but they Did not have the Gigabyte Motherboard, They Had Another cheaper intel one but it was USB 2.0
Actually I think Getting The commercial dell one's are not a bad idea, actually i had totally forgot about that...I Will go and check them out tomorrow..