Tab # limit in IE7?

I usually need a lot of tabs when browsing the web. However, after opening a
certain number of
tabs in IE7 (15-20), the computer begins to behave strangely. If I try to
open a new tab, it just says
"Connecting..." on the top of thw window and does nothing. Right click shows
no menu. Also, no
dropdown lists when pressing the buttons on the taskbar. I can't open
another working IE7 window,
Windows Explorer, and a lot of other programs, unless I close some of the
working tabs. Closing
tabs however doesn't resolve the right click issue and some other problems
(like incorrect load of
Windows Mail). The tabs limit seems to be getting lower as I close them.

I have to kill ieuser.exe and its descendents and all other running
iexplorer.exe to return to normal
operation. Using Process Explorer, I noticed that the iexplorer process
parented by ieuser has no
visible window, the working IE window(s) being the other iexplorer
process(es).

And Norton AV and Spybot didn't detect any malware.

Note: I have IE 7.0.6000.16575 on Vista Home Premium, 1.83 GHz Core2 Duo,
1,5 G RAM.
RAM and Processor usage are within acceptable limits when the problem
described above
happens.

If this is not the right place to ask, please point me to the right one.
If this has been discussed in another place, please give me the link. Or at
least tell me what to search
on the internet, cause I have no idea what keywords to use.

Thanks,
Daniel