I have just been to www.postopia.com and www.nickjr.com to check, and they work ok on both IE and Firefox, (a different browser), on my computer.
So it doesn't seem to be the sites themselves, and as they are reputable young children's games sites, (Nick Jnr is a large site and a tv channel), I wouldn't expect them to have any rogue flash or macromedia software.
So, if you have tried Rod's advice but still have problems, I wonder if your Internet Explorer settings have changed recently.
Have you made any changes to your computer in the last few days/weeks? eg, downloaded and installed XP SP2, or upgraded IE?
If so, those upgrades may have changed the security settings.
On my IE, when I went to both sites, a warning popped up asking if I wanted to allow ActiveX and plugins. As Shockwave and Macromedia are plugins, if your IE security settings are set to automatically disable plugins without asking, then this may be the problem.
In Internet Explorer, goto Tools > Internet Options. Under the Security tab, click the green globe, then click the ''Custom'' button. Under Settings. Make sure;
1] ''Run ActiveX controls and plugins'' is set to Prompt.
2] ''Script ActiveX controls marked safe for scripting'' is set to Enable.
3] ''Java permissions'' is set to Medium safety.
4] ''Active scripting'' is Enabled.
5] ''Scripting of Java applets'' is Enabled.
Click OK, then ''Apply''. Do not click OK out of Internet Options.
Then, click the ''Advanced'' tab, and make sure;
1] If you have Java, ''Java (Sun) - Use Java'' is ticked, (there will be a version number after ''Use java'', depending what version you are using).
2] If you are not using Java, under Microsoft VM, tick each of the 3 boxes. I recommend you download and install Java, and disable Microsoft VM at some stage.
3] Under ''Multimedia'', make sure each box is ticked.
click Apply, then OK. You may have to restart the browser for some settings to take effect.
I hope this helps some. If not, come back and we will see what else can be done.