Human endogenous retroviruses (HERVs) have been an interesting (and expanding) topic of research in evolutionary biology and medicine. A retrovirus has an RNA genome in the virus particle, and integrates with the host cell's DNA upon infection to hijack the transcription/translation machinery and produce copies of itself. For this reason, it is not possible to "cure" a cell of a retroviral infection; instead all of the infected cells must be destroyed or commit suicide through apoptosis, also known as programmed cell death. Common examples of retroviruses include HIV (causative agent of AIDS), HTLV (human T-cell leukemia virus), and the Hepatitis B virus.
Viral infections are most common in somatic cells, where any change in the DNA sequence is not passed along to the next generation. However, any retroviral infection in germ cells (eggs, sperm, and the progenitor cells that make them) could be passed on to the next generation, and over time spread throughout a population. Some HERV fragments are apparently completely inactive (at least as far as we are able to tell currently), and are a possible source for some of the so-called "junk" or non-coding non-regulatory DNA that makes up a significant part of our genome - one estimate claims that 8% of our genetic material is of retroviral origin. Other HERVs have been implicated in various pathological conditions, including multiple sclerosis.
I'm not sure that any human genes have been found that were picked up by a virus, stayed with it for a time (and potentially mutated), then was reinserted back into the human genome, if that's what you're asking. There are certainly many cases of genes and gene fragments (coding regions, promotors, enhancers, etc.) being moved around as a result of viral integration/splicing. http://en.wikipedia.org/wiki/Endogenous_retrovirus#Genome_Evolution is a large, rather well-written section with numerous links to the primary literature showing how our genome has been altered over the years by HERVs.
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