You bought a used computer (Pentium III) in addition to all the garbage it probably has malware on it. If it was me I would format the computer and reinstall Windows.
BEFORE:
Backup your data. Check each disk and make sure all the files are OK. I have found backup disks that had corrupted files.
Find your product keys, Run "Keyfinder Thing 3.1.6" and print out the results.
http://www.download.com/Keyfinder-Thing/3000-2094_4-10561260.html?tag=lst-0-1
Download and run the Belarc Advisor and print out the results.
http://www.belarc.com/free_download.html
Find all your drivers and software and software keys now and develop a plan on how you will reinstall everything.
http://www.thefuhrmans.com/reformat_hd.htm
http://blog.angrypets.com/2006/01/ultimate_prefla.html
http://www.smartcomputing.com/editorial/article.asp?article=articles/archive/l0910/32l10/32l10.asp
Reformat and install. (Cleans drive loses all data)
http://support.microsoft.com/kb/313348
http://www.microsoft.com/windowsxp/using/setup/winxp/install.mspx
http://www.whitecanyon.com/reformat-the-hard-drive-in-windows-xp.php
http://www.smartcomputing.com/editorial/article.asp?article=articles/archive/l0910/32l10/32l10.asp
Reload Drivers, Spyware/ Virus protection, All Microsoft updates, Programs, etc.
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In some cases some people like to overwrite the entire disk before reformatting and reinstalling windows to prevent any old files from being recovered and/or to eliminate any possibility (rare) of malware still being on the hard drive after formatting and reinstalling windows . Any of the following will accomplish this.
http://forums.cnet.com/5208-6142_102-0.html?forumID=5&threadID=194873&messageID=2115918
"You can find out the manufacturer of your HD and then attempt to download the formatting tool created by that manufacturer for their HD's. Most HD Mnufactuer have a Zero Fill Low level formatting tool available to erase their HD products. This will clean any existing OS files from your system as well as any viruses other files etc.."
Darik's Boot and Nuke (DBAN) free
http://www.download.com/Darik-s-Boot-and-Nuke/3000-2092_4-10165154.html
http://dban.sourceforge.net/
DBAN will automatically and completely delete the contents of any hard disk that it can detect, which makes it an appropriate utility for bulk or emergency data destruction.
Active@ Kill Disk - Hard Drive Eraser (Free version/ $29.95 enhanced version)
http://www.killdisk.com/
Free version limited to 1 erasure pass and first hard drive.
Active@ KillDisk - Hard Drive Eraser is powerful and compact DOS software that allows you to destroy all data on hard and floppy drives completely, excluding any possibility of future recovery of deleted files and folders. It's a hard drive and partition eraser utility.
WipeDrive by White Canyon $39.95 (no free version)
http://www.whitecanyon.com/
Completely wipe and erase all the information from a computer before selling or discarding it. WipeDrive makes it easy.
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AFTER you reinstall windows
Microsoft Update "Custom Mode" install everything. Reboot and repeat until there is nothing left to install.
http://update.microsoft.com/microsoftupdate/v6/default.aspx?ln=en-us
In addition to the Microfoft XP Windows Firewall you will need full time (active) virus protection and full time spyware protection.
All the following programs are free.
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Full Time (Active) Virus Protection
If you do not have active virus protection, install only one. All are excellent.
AVG Antivirus 7.5 Free Edition
http://free.grisoft.com/freeweb.php/doc/avg-anti-virus-free/lng/us/tpl/v5
http://www.download.com/AVG-Anti-Virus-Free-Edition/3000-2239_4-10669237.html?tag=lst-0-1
or
Free antivirus - avast! 4 Home Edition
http://www.avast.com/eng/avast_4_home.html
or
AOL Active Virus Shield
http://www.activevirusshield.com/antivirus/freeav/index.adp
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Full Time (Active) Spyware Protection
Windows Defender
http://www.microsoft.com/athome/security/spyware/software/default.mspx
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Install the following five programs and run weekly or at least monthly. You need all five. They will greatly increase your protection. They are not a substitute for full time spyware and virus protection.
Ad-Aware SE Personal (update + full scan)
http://www.lavasoftusa.com/products/ad-aware_se_personal.php
Spybot Search & Destroy (update + immunize + scan)
Do not enable Tea Timer and SDHelper
After installation: update + scan + immunize
http://www.safer-networking.org/en/mirrors/index.html
SpywareBlaster: Update then open and click “enable all protection”.
http://www.javacoolsoftware.com/spywareblaster.html
SUPERAntiSpyware free version: (update + scan)
http://www.superantispyware.com/
CCleaner: Do not install toolbar option
Removes tracking cookies, unneeded files, history
In options.
Set to run when computer starts.
Place cookies you want to keep in save list
http://www.ccleaner.com/