I want to know that if a HDD with clicking sound(which means probably the HDD is faulty) which although runs perfectly fuine with occasional disk errors(sometimes) actually cause your comuputer to slow down. Like in my base I am running Windows Xp on Pentium 4 with 1 GB ram, I think its the HDD whihc is making the programs to respond slowly as when the computer gets stuck there are alot of clicking sounds from HDD. I am thinking of getting the HDD replaced, in any case. But is it the cause of the computer slowing down esp with LAN plugged IN(Internet)
I would definitely replace your HDD ASAP and more importantly back up anything of value now. Clicking Hard drives are a ticking time bomb (excuse the pun)
With regards to your question, yes this could be a cause of the slowdown especially as you say "when the computer gets stuck there are alot of clicking sounds from HDD" However the line "esp with LAN plugged IN(Internet" makes me wonder if there are other issues.
It could be the drive causing the performance problems, but if the drive is fairly active it can be a sign of two things: either you don't have enough RAM in your computer or the drive is dying (or both). Once your system consumes all of the physical RAM, it uses space on your HDD to create "virtual memory". What that means is a portion of the disk is used to fool your computer into thinking it's actually a part of the physical memory installed so that it can carry on operating. The problem is that HDD access is far, far slower than accessing physical RAM. This itself can cause a significant performance decrease.
An easy way to tell if you don't have enough RAM is to open Task Manager and look at the amount of memory that's being used. In your case, if that number is going over 1GB, then you know you also don't have enough RAM.
However, for now, your main concern should be protecting your data and getting ready to replace your drive. I've had clicking drives in the past and they really don't tend to last very long. The only way to backup everything properly is to make a list beforehand. Don't wait for anything to get worse. The faster you get on top of it, the less chance there is of you losing something important.