Your PC could be dealing with a latency issue. Latency is a delay, and that delay may be causing your "lag". I am only talking about "system latency". Network latency could be your problem, but I would try this first.
*Disclaimer: We are assuming that your network speed/bandwidth has not been fluctuating or reduced. AND We are assuming that you do not have any PC hardware or malware problems*
Basically speaking, if you have tons of programs (processes, or services) running, they are all competing with your PC's resources for attention. If there are too many processes running at the same time, one or more of the process "commands" or requests to the PC will have to "wait" until the previous command has completed (think about your busy chatroom when the random n00b complains that you didn't answer his Q fast enough, that's latency). This "wait" time is latency, and could cause your lag. Another cause of latency is where a poorly written program is causing havoc and not playing well with the other kids in the sandbox. I started having problems when I updated FF AND a USB driver the other day, so I've been dealing with this exact same thing.
This only applies to Windows O/S:
1: Create a new user account. Do not make it an administrator account, power user should be fine
2: Edit the start-up group to the bare minimum. Look at your system tray in the bottom right. Is it full of icons? Those icons represent running processes.
3: Since this new user account is just for camming, you decide what programs you need running. I would start off with the bare necessities: model software and/or browser for web client; cam s/w; jukebox s/w, and what ever else you really "need". Do you really need IM and mail clients running? Do you need tweetdeck (or similar) running? Since this is a test, just run only what you need to cam.
4. When you have done all this, log off & log on (or reboot) into the new "test" account. Do NOT use the Windows "switch user", as that uses PC resources.
5. Test it out, give it a dry run. If everything is working OK, you can add back optional items, one at a time and go from there. Doing this, you should be able to isolate the problem.
It should also go without saying that what ever browser you use should be updated. For this test, I would dis-able ALL browser themes, BHOs, frivolous add-ons, and all browser toolbars. The same is to be said for your flash/java/Windows; and cam/video drivers: update them all.
If you are not sure how to do this, there are plenty of free resources on the web. Google "how to create new Windows account" and go from there. Most of the results will be detailed with step by step screen shots. Get some of your PC literate friends to help. This should really only take 30-60 minutes (or less), and ideally you don't want to start this 10 minutes before you go on cam.
There are many other troubleshooting tricks you could do, but this is just a basic starting point. Good luck!