Are you connecting via wire or wireless? Does your setup have wireless available? Has the wireless been locked down (password protected)? Do you have neighbors close enough to be using your connection? If enough people pile onto your wireless, it will slow you way down.
Have you tried having a friend come over with a laptop to see if another machine acts the same way as yours does in your environment? If it seems to work fine and yours does not, that would tend to limit the problem to your box - If it acts the same as your box, then it cannot be your box, but rather something in the wiring/modem/router... The reverse is also true - if you take your box to another environment and it works fine, the box itself is eliminated from the possible problem.
Try to determine if there is any time patterns - if it is always bad around say, 10:30 and 4:00, but is usually good at say, 2pm and 10pm, then it is probably throttling performed by your DSL provider (limiting their users to preserve bandwidth)... if that seems true (and they will lie through their teeth and tell you they don't throttle), the only thing to do is try a different provider (cable would be nice).
Try unplugging all external peripherals - especially USB devices - Since USB is a 'serial port' acting like a 'network share' a malfunctioning printer (or whatever) can send goofy signals into networking that really bungs things up.
How is the power? are you on a battery? Poor power can do crazy things.
Nuther thing:
IF router/modem (NOT only modem)... Was the modem reset? If it was, it needs to be bridged to work with the router. it is a simple software setting, but it will do some weird things if it is not.
IF router, AND router is ‘G’ series, AND running WinVista, Win7, Win8, Router will be the problem... MAY be able to be fixed with a firmware upgrade, if not, replace the router with an ‘N’ Series.