Have you done Disk Cleanup lately?  Type "cleanmgr" into Start/Search and
hit Enter.

Then use an online virus scanner to find out if you really have a virus.
You well may not but may have other issues.  Some scanners to use are: (they
all take a while)
http://www.microsoft.com/protect/products/computer/safetyscanner.mspx
http://us.mcafee.com/root/catalog.asp?catid=free#catalog_mobile
http://security.symantec.com/sscv6/default.asp?productid=symhome&langid=ie&venid=sym

1. CCleaner - Free
Cleans temporary internet files, cookies, history, recent urls, application
MRUs, etc. ...
http://www.filehippo.com/download_ccleaner/
If Windows Defender is utilized go to Applications, under  Utilities
uncheck "Windows Defender".

2. SuperAntispyware - Free
http://www.superantispyware.com/superantispywarefreevspro.html

3. Kaspersky® AVPTool
http://avptool.virusinfo.info/en/
http://downloads5.kaspersky-labs.com/devbuilds/AVPTool/

After the software (#2) is updated, it is suggested scanning the system
with both SAS and KAVPT in Safe Mode.

Problem still not fixed?

Download and execute HiJack This! (HJT)
http://www.trendsecure.com/portal/en-US/threat_analytics/HJTInstall.exe

Please, do not post HJT logs to this newsgroup.
Fora where you can get expert advice for HiJack This! (HJT) logs.

NOTE: Registration is required in any of the below before posting a log

http://www.theeldergeek.com/forum/index.php?s=2e9ea4e19d3289dd877ab75a8220bff6&s
howforum=29
http://www.thespykiller.co.uk/index.php?board=3.0
http://www.bleepingcomputer.com/forums/forum22.html
http://castlecops.com/forum67.html
http://forums.tomcoyote.org/index.php?showforum=27
http://www.5starsupport.com/ipboard/index.php?showforum=18
http://www.spywarewarrior.com/viewforum.php?f=5

Good luck :)

Two days ago I 'checked' Require Users to Press CTLALTDEL at logon.  I
was surprised that I had to press CTLALTDEL after the id-pw is entered.
Somehow I thought it would be before.  By the next day I had logged on a
couple of times and when I tried to get it up from sleep I got this:

*Your system is running low on resources.  You cannot log on as a new
user.  Please use an account that has already been logged on. *

I eventually got back on my Administrator account and used Event
Viewer to see what there is to see.

ACPI: ACPI BIOS is attempting to write to an illegal PCI operation
Region (0x5).  Please contact your vendor for technical assistance.

Something I vaguely remember about PORT A being unavailable or
unaccessible, shadow copy of Cdisk eliminated for such and such reason.
I also remember that Kaspersky was mentioned but isn't that an anti-spy?
Then it froze while I was in Event Viewer.  I brought it back up and
it was continuing to generate alarming msgs so I shut it down and now
I'm afraid to bring it up again.  (Just heard about PostcardFromHallmark
Virus creaming the hard disk.)  

What should I do first?
Find out what ACPI is.  I noticed one guy fixed a system freeze by
disconnecting ACPI.  
Find what exactly is running.
Look for something else in the Event Log
Run some diagnostic tool(s)
(Bring it up in Safe Mode to avoid it running self-destructive-ly)
Retrieve system info -HP Pavilion Dual Core WHomePremium Vista 32-bit

After reading  'Anti Virus Protection' I don't believe I have any but
AVANT seems recommended.
FYI this was an officedepot display model.

Appreciate any suggestions.