Encrypted data

If you are referring to data from the Internet, take a look at Network Connections/"Your connection name"/right click/Properties/Security.  Is Typical checked (that is normal for most people)?

The readability is the opposite to what it should be.  How do you know it is encrypted, warning dialog?  Not being able to read a document is not necessarily a sign of encryption.  If the internet security setting is OK, post some more details about how you can't read the data.

Some of the files are on both My Documents and a floppy and I get the same results.  I have XP Home, so I am more mystified as to why and how they get encrypted.  No one seems to know this, even Dell.

The symptoms you describe suggest the data is not encrypted coming into your pc, but it being encrypted on it.  When you refer to using a floppy and reading on another pc, presumably that data never went into My Documents?

The following only applies if you are using XP Pro, encryption is not available as standard in XP Home:

In Explorer, go to the offending folder My Documents, right click, Properties, General Tab, Advanced.  Is "Encrypt contents to secure data" checked?"  If so, uncheck. You may need a password that you obviously don't know!  Make sure you are logged in as Administrator - it may give you more options to undo any encryption.

Compressed files cannot be encrypted so that may explain why some files are encrypted and others are not.

I'm out of ideas for now.  As shots in the dark you could try deselecting TLS in the Internet Options/Advanced tab/security section if it is checked and see if subsequent downloads are affected (will use SSL instead).  Also, check for malware. 

Keep a log on exactly which sites are problems.  Try emailing them to see if they have suggestions - commercial sites may deal with thousands of downloads/correspondence a day and are likely to have seen every problem.