Most common cause is probably just the fact that you have too many applications/processes running which are using up your RAM.
First thing to do is check you don't have any unwanted "nasties" lurking on your PC........run Ad-Aware first, then also run X-Cleaner and afterwards Spybot. If you don't have them already they are free to use and available from
Major Geeks ..... link takes you to main download page, all 3 programs are listed there (Ad-Aware SE Personal, Spybot Search and Destroy and X-Cleaner) and shouldn't take too long to download. Between them they should pick up most malware/spyware/crimeware programs on your PC. To be really safe I would recommend using CounterSpy (available at same site)....this used to give a free 15 day trial, however they have now changed it to a 30 day money back trial which means you need to pay for it first!! Having said that, it is only (!!) $19.95 and is worth every penny......probably the most efficient malware remover around, it also gives you real time protection which stops the nasties from even reaching the hard drive!!
If you are still having problems, there should be an error code which appears when you get the "run time error" message appear. Make a note of the code before you restart your PC. After re booting go to
http://www.microsoft.com go to support (listed on lefthand side of page under "resources") and enter the error code in the searchbox. This SHOULD bring up detailed information on what is causing the problem and how best to sort it out. I would recommend you print out the information and read it several times before acting on the instructions (it can be quite technical, but if you follow the instructions exactly you shouldn't have too much trouble!!)