My understanding of a 'decompression bomb' is a file that is highly compressed such that when it is uncompressed, it might consume a considerable amount of disk space. It could be something like a ZIP file or an compressed installation file for a certain
program for example.
avast! likes to detect them, but they are not necessarily infections on your system - hopefully it is just more like an informational message.
However, it
could be malicious:
http://en.wikipedia.org/wiki/Zip_bomb
Since you did not tell us what the name(s) of the file are or where they are located (you might want to consider doing that) nobody will be able to tell you if these files are something you need or not.
Once you identify the file and who it belongs to, you can decide what to do about it. Then it also might depend on where the file is... If it is a temp folder, you can clean your temp folders to get rid of them.
If you ever want a second opinion on any suspicious files here are some resources that will scan them:
http://www.virustotal.com/ (about 40+ scanners)
http://virusscan.jotti.org/en (about 20+ scanners)
For virustotal, if you get a message:
File has already been analysed
Click the Reanalyze file now button and wait. You might be in a waiting queue depending on how busy they are. If the files are large, it may take a while to upload them.
You should see some progress indicator and the result will be displayed on the browser screen from A-Z when all the scans are done.
Then, if I were you, I would supplement your avast! with these popular and respectable free scanners:
No matter what else you are using for malicious software protection, do this:
Download, install, update and do a quick scan with these free malware detection programs (not at the same time) and remove any threats:
Malwarebytes (MBAM):
http://www.malwarebytes.org/products/malwarebytes_free
SUPERAntiSpyware: (SAS):
http://www.superantispyware.com/
SAS will probably report a bunch of tracking cookies and you can just let it delete them.
Do a full scan once in a while when you have more time (perhaps hours).
They can be uninstalled later if desired.
