Recently i upgraded my internet connection and downloaded files from rapidshare at 50+ kb/s and megaupload at 40kb/s. Two days ago suddenly my internet speed became slow around 25kb/s and changing not consisten like before. i called my isp and they said they would check line.
At night the downloads returned to its fast normal speed and i thought the prb was solved. But in morning and during the whole day the speed became slow again. so i gave them a call. the guy asked me to download 3 files at same time and when i did the total speed added up to around less than 60kb/s.

Reinstalling the OS shouldn't be necessary, especially if the problem is intermittent. Call your ISP tech support back and have them escalate the problem. If you sound like you know what you're talking about when you tell them that this is NOT a problem with your OS, then they may reconsider their diagnosis. This was done to put the ball back in your court and get you off of the phone. Don't tolerate that. Be polite, but firm. Being a little indignant due to their suggestion wouldn't be inappropriate.
You didn't mention what type of connection that you're using. If it's cable, then your bandwidth may be affected by the number of people online. If it's DSL, it could be a physical cabling problem. Interference from home appliances, including other phones being used, distance from central office, etc.. If DSL, be sure that filters are connected and working properly. However, it really sounds as though your connection is being throttled back during the day, (except when you were requested to DL three files when you experienced the best performance ever! That's no coincidence!)
There are several programs that are designed to help you setup your network and configure your PC to connect to it.How are you connecting to your router, ethernet or wireless? It could be your network interface card (NIC).
ou can try to test the speed of your connection at www.speedtest.net
Also, to get a better gauge of your download speed try downloading something big from say microsoft.com which usually have a bigger pipe than most p2p users
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http://www.4tech.info/trick-zone/how-to-increase-internet-speed-without-any-software/
keep writing buddy! you re good in this! :)