Vista - Problems with wireless

Good evening all! And sorry to start my membership in the forum on a question...

You see, I am building my first network for me and my girlfriend. I bought a cheap (sweex lw050) router, which is to provide a wireless connection between two pc's and the internet. My pc runs windows XP and has no problems whatsoever. My girlfriend's pc runs Vista on an x086 machine, and her computer has connection problems.

The problem with the router is only when it is encrypted, in which case it does not connect well to the router at all (limited connectivity):
PC ---- ! ----- Router --------- internet
These problems vanish if the network is not secure. However, even if it isn't, it is slow and patchy, sometimes dropping off entirely. If I insert a normal LAN cable, everything works fine.

Any ideas?

edit: To make it crystal clear, the XP running machine is encountering no such problems.

As an aside, my girlfriend has a dell.