The "low memory warning" has nothing directly to do with the amount of RAM that's in use. It has to do with running out of commit limit, which is the maximum possible amount of private pageable address space. The commit limit on a Windows system is size of usable RAM + your pagefile size. If you've disabled the pagefile, as apparently you have, then it's just the size of usable RAM. It doesn't matter how much of the RAM is "in use"; the commit limit (you'll see this on the Task Manager screen) stays the same. If a program's request for private virtual memory, added to the current commit charge (this also is on the TM screen), totals to more than the commit limit, the request for virtual memory will fail, and you get the error popup.
The cure is to enable your pagefile again... or add more RAM, if you can.
n.b.: Windows systems do not "freeze very frequently". I've never disabled my pagefile (except for test and demo purposes) on any Windows NT-family system, going back to the pre-release beta of NT 3.1, and the only times I've ever seen them "freeze very frequently" were due to completely different problems. Re-enable your pagefile and then fix the other problems.