Data Flowing Upstream Mirroring Downstream While Browsing?

using task manager in XP it is clear that while browsing data is being uploaded mirroring the data downloaded..ie in a given period if 18mB comes downstream (just surfing) then 11mB goes upstream. The graph in task manager shows that the peaks and troughs of the data upstream and downstream exactly correspond and watching the bytes tick over confirms that data goes out for every data coming in.I assume that this should not happen? I realise ip protocols have some kind of error detection that may require uploading data, but the amount sent seems excessive! From my limited understanding of networking and running wireshark it looks like that when packets come from an ip on the web ( i use the terms web/internet interchangeably ) then packets are sent out to the same ip ... using TCP and HTTP ( I don't really understand them ). The info for one such packet going out is "Continuation or non-HTTP traffic" using the HTTP protocol, which sounds a bit contradictory. I regularly run virus scans and rarely find anything. The cpu regularly maxes out and its usually something to do with firefox ( I've heard of buffer overflows but i assume the problem is a relatively old processor and hardware).The browser is firefox. OS is XP.Coincidentally, the pc was recently rebooting after crashing until I disabled "restart on system failure" which prevented the crashes ( if they were crashes and not just the system reacting to an error ). Again, that is a bit suspicious but maybe not. Spybot, bit defender quickscan,avira, zone alarm, malwarebytes etc haven't flagged anything up.Maybe the router is not configured properly. As with all these things, there will be some simpler things to start with to diagnose this issue (if there is one ) but I don't know what they are.The pc uses wifi to connect. The isp is not the best and the speed is pretty bad for adsl. Every couple of days the router needs rebooting because it stops giving out ip's.