[ordinaryguy]

I was minding my own business surfing the net when suddenly an unsolicited download of something called WinAntiSpyware started. I stopped it immediately, but it put a window on my desktop that's always on top of everything and I can't get rid of it. It says "Attention! Activity by Unknown Process has been detected! Name: yayywxw.dll Path: C:\WINNT\system32\yayywxw.dll". Also there's now an icon on my desktop for WinAntiSpyware, and it's in my list of programs. I tried to uninstall it, but the uninstall process hangs up part way through and nothing happens. Is this a legitimate antispyware program? Why would it suddenly start downloading and install itself without being asked? How can I get rid of it? At various points in the process of trying to get rid of it several other threatening pop-up windows have opened, all offering to save me from dire threats if I will "REGISTER NOW. Most of these windows say Internet Explorer at the top, even though I'm using Firefox as my default browser. It all seems really sinister to me and it's pissing me off no end.

Here's the url for a tab that opens uninvited and directs me to download WinAntiSpyware:
Security Update

[iAMfromHuntersBar]

It seems like a very nasty bit of spyware!

Here's a couple of sites with removal instructions;

Detect & Remove WinAntiSpyware » 411-Spyware.com
How To Remove Winfixer / Virtumonde / Msevents / Trojan.vundo.b

I hate these programs!

Some have been designed now so that when you click anywhere in the window it accepts the download... or so I've heard!

[Curlyben]

Can you say SPYWARE!

Sounds like a classic infection.
Time to do some serious maintenance on your machine:

Here's my usual C&P for cases like this:

When was the last time you did some serious maintenance on your system ?

System maintenance includes:
Defrag
FULL patching
Virus scanning
Spyware scanning
removal of rubish files

Defrag is an inbuilt function in XP.
Open My Computer > Right click C: > Tools Tab and its there.

To make sure everything is running fine run both anti virus and anti spyware apps in normal AND safe modes. (make sure that they are updated first ! ;)) (AVG is good and free AV)
(A couple of good removal tools are Spybot and Adaware)

ALso an on line virus and spyware scanner is Trend Housecall

Just a note: actively running two AV's on one machine can cause problems.
So if you are thinking about it make sure your current one is disabled first.
Same thing applies to online scanners as well.

Removal of junk files is easy with CCleaner a free app that does exactly what

If all this fails then a repair install may be in order:

Here's the official Micro$oft way of doing it,
and Here's one with screen shots.

Basically this installs windows over the top of your current setup.
So you don't loose any information you already have.

This lot should resolve your issue.