I use Vue 5 Infinite, and everything has been just fine until recently. For no apparent reason I can think of, I have noticed that when I go to render an image, whether it's a preview render on screen, main view, or a full render to hard disk, I am stuck there until the render stops or I stop it. I can't go do anything else on the PC. Even if i already have other windows open, I can't use them. It's like Vue has taken control until rendering finishes. 

Has anyone experienced this or know what is going on?

Having just gone through this I can give you a couple of suggestions.  Run a RAM test to make sure your RAM is okay.  Defrag your hard drives and then make sure your fans are clean.

If the scene is complex I suggest using the external render engine.  It's slower than the internal renderer but it will just keep chugging along until the render is complete.

I ended up having to replace my heat sink because it was faulty and Vue wouldn't render anything because of thermal shutdowns.  I spent a lot of hours going back and forth with e-on tech support, including sending them the scene file, until we started going into the hardware and we got it figured out.

A free Memory Test prog you can find at www.memtest86.com takes several hours.  You should let the computer warm up to see if as Victoria says, you are having a heat problem, before testing the memory.  

When booting look to see what the CPU temp is If you have setup info.  Should be 120 and less to 90 F after two hours operation.  150 to 200 is hot, and may cause the computer to lock up.

As Victoria mentions, clean everything, even the Power supply if possible.  If you cannot reach the inside, use compressed canned air with a long soft haired  artist brush.  

For the CPU, get new silicon heat dissipating paste like Artic Circle, the better quality the better heat dissipation. 

If none of the above and defragging the hard drive does not work, then back up your Vue materials folders and then reinstall VUE.

My CPU generally runs about 115 degrees F after warmup and it's consistently lower than 120 even when rendering a complex scene.  I downloaded a utility from my MB manufacturer that allows me to monitor my CPU temp.  

When I replaced the heat sink I did buy some better thermal gel and that's made a big difference, as well, so keenart is quite correct.

Thanks Victoria and keenart. Cleaning the PC is something I do as regularly as it needs. I downloaded a memory test program and that seems to indicate that it's fine.  Everything else seems ok too so it may well be a graphics card problem. (It is old and slow and I may just have run it into the ground).

Thanks for your information and help.

Based on the information you have provided, there are two possibilities, provided your graphics card has more than 64 megs of memory;

1. Your motherboard B+ circuit is showing a sign of age, in which case replacing the motherboard is the only solution.
2. The other possibility is the Power Supply itself. It is not uncommon for a power supply to lose viability on a machine that is consistently used for Rendering 3D graphics.

The graphics card should not be the issue, unless Vue is consistently crashing means bad drivers. Otherwise, a general slow down of system and applications usually indicates overheating, damaged CPU, Memory, or lack of an adequate supply of power. The CPU and the OpenGl software coupled with the memory and sometimes swapping out to disk are the main source of Rendering. The graphics card simply displays the results of the Render to screen, but does not usually do the Rendering itself.

Therefore I would swap out the power supply first. Find out what wattage your motherboard, CPU, and Graphics card require, i.e. 350 watts, and add an additional 10 to 25% more wattage and get the latest 20 or 24 pin power supply. Which ever pin configuration your motherboard requires? 

If the power supply is not the problem, then it is time to upgrade the board or better yet the system.  

Again, before you try all of the above, I would back up your Vue Materials Folders, and uninstall and reinstall Vue.  In addition Check the Administrators, Events Viewer to see if there are any Errors or warnings concerning sytem or application problems which may be related to the Operating System..