Occasional blocking of access to the net

Every so often I log on to find that although my D-Link router is connected, (or appears so from the little icon in the toolbar), I simply cannot get on to the net and consequently of course cannot receive my emails. I know its not my email programme which is Turnpike nor is it MailWasher because when I try to log on to Google from the desktop I am also denied access. To put it simply, I am blocked from getting on the net. I have had a technical check-up with my IT company who managed to "cure it" but are unable to tell me how they did it. Of course it has recurred several times since and caused me a lot of trouble and grief. One piece of information I can give to help with a solution is that I have a definite and guaranteed method of coming out of it, I simply pull out all the plugs in the wall, (i.e. switch off), all the current to "everything" and hey presto, when I plug in everything and restart my PC all is well. Re-booting my PC alone will not cure it, I have to turn off the current, leave it a few seconds and turn on again.

Can anyone tell me what is wrong and how to stop it happening?

A few details, I use XP Home edition with Service Pack 2 installed and a Pentium 4 (2002) with 512 RAM. Any further details you need I will supply. I regularly run SpyBot, Lavasoft Ad-Aware and have AVG Free edition Anti-Virus etc. I use Disk Keeper and carry out all sorts of maintenance checks regularly. I would be so grateful for a solution to this tiresome problem, thank you. -- -- Derrick Fawsitt