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I am running Symantec antivirus on a Windows XP machine. The amount of free disk space I have in C: has been dropping every day for the past 15 days.

I have been emptying my Recycle Bin, and I only have important software installed. I'm using a 40GB disk, but I keep getting warnings about low disk space. I've scanned my machine at boot time with Avast, but it didn't solve the problem.

Could this be a virus? I don't want to reformat the whole computer, because it would be hard to restore my configuration.

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This may be obvious, but have you emptied your recycle bin recently...? – Majenko Apr 10 '11 at 19:51
"Where is my free space going?" has been covered again and again and again and again and again on SuperUser. It's not necessarily, or even often, a virus. Don't leap to conclusions like that. Your partition size is remaining constant, by the way. You are talking about the remaining free space. – JdeBP Jun 5 '11 at 14:21
I edited your question to reflect what JdeBP said about partition size and free disk space not being the same thing. If you actually are somehow seeing your partition size changing, I apologize. – Pops Nov 14 '11 at 6:11

A good, easy way to see, visually where all that space is going is windirstat - you can use that to more intelligently track down and clear whatever's taking up that space. If you can't see where the space had gone, i'd suspect a rootkit, and someone using your hard drive for unauthorised storage - but that's not hugely likely. Good rootkit scanners are gmer and sophos.

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An alternative to windirstat, TreeSize Free. – therube Jun 5 '11 at 11:41

If You think you are Infected

Follow the order given below to disinfect your PC

1.) On a PC that is Not infected, Make a boot AV disc then boot from the disc on the Infected PC and scan the hard drive, remove any infections it finds, I prefer the Kaspersky disc myself. The New 2010 Kaspersky disc can update the AV dat files if you are connected to the internet at the time of scan and is suggested to update before the scan.

2.) Then: Install free MBAM, run the program and go to the Update tab and update it, then go to the Scanner Tab and do a quick scan, select and remove anything it finds.

MBAM is not AV software like Norton AVG etc, it is an on demand scanners that only scan for nasties when you run the program and will not interfere with your installed AV, MBAM can be run once a day or week to ensure you are not infected. Be sure you update them before each daily-weekly scan.

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also, if mbam won't install, it might be a sign of a virus - some malware writers explicitly block mbam – Journeyman Geek Apr 11 '11 at 0:27

It may be because of the automatic updates that win does. The binary files are stored in a temp location. Check to make sure that you do a CCleaner after a successful windows update.

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