Why do large companies always use rubbish hardware and software in their PC's?

I work for a leading insurance company, and being a total computer geek, I am constantly frustrated by the rubbish machines we have to work on. The machines themselves are HP but with pathetic AMD Athlon single-core processors, equally-pathetic integrated graphics and a measly 512MB of RAM! We have constant system crashes because we all have to have between 10 and 20 applications open at the same time (including Word, Excel and Powerpoint presentations), yet the company never does anything about it. And as for the software - they're still using IE6! I mean, who the hell uses that awful browser these days?! It should at least be IE7 (though Firefox is far superior than any browser made by Microsoft). I just cannot understand why a company that makes such huge profits would have such rubbish equipment. And why is it never kept up to date?! My own laptop is far more powerful...and I only use it to surf the Net! It drives me mad that I have to work with such inferior equipment every day.