It could be many things, however those that come to mind are:
1. Spyware infection. Under most circumstances IE really shouldn't lock up, however if there's a spyware infection that messes with IE it can become very unstable -- the same goes for other web browsers that spyware can infect as well.
2. Some toolbar, or BHO that's making IE unstable. You can see and even disable BHO's that are installed in IE by using:
Internet Options->Programs->Manage Add-ons
3. You many need to run IE's maintenance tools: Delete Cookies, Delete Files, and Clear History. Then run CCleaner so that it can mark the index.dat files for deletion on the next system restart.
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You can download a third-party freeware browser that isn't easily infected by spyware such as
Opera,
Mozilla Firefox, or Mozilla Suite.